Cross-reference concept exercise file information (exercism#1320)

[Docs] Cross-reference concept exercise file information

reference/implementing-a-concept-exercise.md

@@ -19,7 +19,7 @@ As this document is generic, the following placeholders are used:
 
 Before implementing the exercise, please make sure you have a good understanding of what the exercise should be teaching (and what not). This information can be found in the exercise's GitHub issue. Having done this, please read the [C# concept exercises introduction][concept-exercises].
 
-To implement a concept exercise, the following files must be created:
+To implement a concept exercise, the following files must be added:
 
 <pre>
 languages
@@ -28,11 +28,11 @@ languages
         └── concept
             └── &lt;SLUG&gt;
                 ├── .docs
+                |   ├── after.md
                 |   ├── instructions.md
                 |   ├── introduction.md
                 |   ├── hints.md
-                |   ├── source.md (required if there are third-party sources)
-                |   └── after.md
+                |   └── source.md (required if there are third-party sources)
                 ├── .meta
                 |   |── config.json
                 |   |── design.md
@@ -42,32 +42,50 @@ languages
                 └── &lt;NAME&gt;Tests.cs
 </pre>
 
-## Step 1: adding track-specific files
+## Step 1: Add code files
 
-These files are specific to the C# track:
+The code files are track-specific and should be designed to help the student learn the exercise's concepts. The following C# code files must be added (not necessarily in this order):
 
-- `<NAME>.cs`. the stub implementation file, which is the starting point for students to work on the exercise.
-- `<NAME>.csproj`: the C# project file.
-- `<NAME>Tests.cs`: the test suite.
-- `.meta/Example.cs`: an example implementation that passes all the tests.
+### Add`<NAME>.cs` file
 
-## Step 2: adding common files
+**Purpose:** Provide a stub implementation.
 
-How to create the files common to all tracks is described in the [how to implement a concept exercise document][how-to-implement-a-concept-exercise].
+- The stub implementation's code should compile. The only exception is for exercises that introduce syntax we _want_ a student to define themselves, like how to define a class or property. In this case, insert a descriptive TODO comment instead of providing stub code (see [this example][todo]).
+- Stub methods should throw a `NotImplementedException` which message contains the method to implement (see [this example][not-implemented]).
 
-## Step 3: add analyzer (optional)
+For more information, please read [this in-depth description][stub-file], [watch this video][video-stub-file] and check [this example stub file][example-stub-file].
 
-Some exercises could benefit from having an exercise-specific [analyzer][analyzer]. If so, specify what analysis rules should be applied to this exercise and why.
+### Add`<NAME>Tests.cs` file
 
-Skip this step if you're not sure what to do.
+**Purpose:** The test suite to verify a solution's correctness.
 
-## Step 4: custom representation (optional)
+- [xUnit][xunit] is used as the test framework.
+- Only use `Fact` tests; don't use `Theory` tests.
+- All but the first test should be skipped by default (check [this example][skip-fact]).
 
-Some exercises could benefit from having an custom representation as generated by the [C# representer][representer]. If so, specify what changes to the representation should be applied and why.
+For more information, please read [this in-depth description][tests-file], [watch this video][video-tests-file] and check [this example tests file][example-tests-file].
+
+### Add`<NAME>.csproj` file
+
+**Purpose:** The project file required to build the project and run the tests.
+
+For more information, check [this example project file][example-project-file].
 
-Skip this step if you're not sure what to do.
+### Add`.meta/Example.cs` file
+
+**Purpose:** The idiomatic example implementation that passes all the tests.
+
+For more information, please read [this in-depth description][example-file], [watch this video][video-example-file] and check [this example file][example-example-file].
+
+## Step 2: Add documentation files
+
+How to create the files common to all tracks is described in the [how to implement a concept exercise document][how-to-implement-a-concept-exercise].
 
-## Step 5: format code
+## Step 3: Update list of implemented exercises
+
+- Add the exercise to the [list of implemented exercises][implemented-exercises].
+
+## Step 4: format code
 
 All C# code should be formatted using the [`dotnet format` tool][dotnet-format]. There are two ways to format your C# code:
 
@@ -81,6 +99,18 @@ Open a command prompt in the `language/csharp` directory and then run:
 
 - `./format.ps1 <SLUG>`, where `<SLUG>` is the exercise's directory name.
 
+## Step 5: Add analyzer (optional)
+
+Some exercises could benefit from having an exercise-specific [analyzer][analyzer]. If so, specify what analysis rules should be applied to this exercise and why.
+
+_Skip this step if you're not sure what to do._
+
+## Step 6: Add representation (optional)
+
+Some exercises could benefit from having an custom representation as generated by the [C# representer][representer]. If so, specify what changes to the representation should be applied and why.
+
+_Skip this step if you're not sure what to do._
+
 ## Inspiration
 
 When implementing an exercise, it can be very useful to look at already implemented C# exercises like the [strings][concept-exercise-strings], [datetimes][concept-exercise-datetimes] or [floating-point numbers][concept-exercise-numbers-floating-point] exercises. You can also check the exercise's [general concepts documents][reference] to see if other languages have already implemented an exercise for that concept.
@@ -103,3 +133,25 @@ If you have any questions regarding implementing the exercise, please post them
 [reference]: ../../../reference
 [dotnet-format]: https://github.com/dotnet/format
 [allowing-fork-pr-changes]: https://help.github.com/en/github/collaborating-with-issues-and-pull-requests/allowing-changes-to-a-pull-request-branch-created-from-a-fork
+[implemented-exercises]: ../exercises/concept/README.md#implemented-exercises
+[video-stub-file]: https://www.youtube.com/watch?v=gkbBqd7hPrA&t=1171
+[video-tests-file]: https://www.youtube.com/watch?v=gkbBqd7hPrA&t=1255
+[video-example-file]: https://www.youtube.com/watch?v=gkbBqd7hPrA&t=781
+[example-stub-file]: ../exercises/concept/strings/Strings.cs
+[example-tests-file]: ../exercises/concept/strings/StringsTests.cs
+[example-example-file]: ../exercises/concept/strings/.meta/Example.cs
+[example-project-file]: ../exercises/concept/strings/Strings.csproj
+[skip-fact]: ../exercises/concept/strings/StringsTests.cs#L11
+[xunit]: https://xunit.net/
+[not-implemented]: ../exercises/concept/strings/Strings.cs#L5
+[todo]: ../exercises/concept/basics/Basics.cs
+[stub-file]: ../../../docs/concept-exercises.md#stub-implementation-file
+[tests-file]: ../../../docs/concept-exercises.md#tests-file
+[example-file]: ../../../docs/concept-exercises.md#example-implementation-file
+[video-stub-file]: https://www.youtube.com/watch?v=gkbBqd7hPrA&t=1171
+[video-tests-file]: https://www.youtube.com/watch?v=gkbBqd7hPrA&t=1255
+[video-example-file]: https://www.youtube.com/watch?v=gkbBqd7hPrA&t=781
+[example-stub-file]: ../languages/csharp/exercises/concept/strings/Strings.cs
+[example-tests-file]: ../languages/csharp/exercises/concept/strings/StringsTests.cs
+[example-example-file]: ../languages/csharp/exercises/concept/strings/.meta/Example.cs
+[example-project-file]: ../exercises/concept/strings/Strings.csproj