[Go] Concepts needed to solve the v2 two fer exercise

languages/go/reference/exercise-concepts/two-fer.md

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+# Two fer
+
+## Exercise
+
+Given a name, return a string with the message: "One for X, one for me.", where the X can be provided and defaults to "you" if not given.
+
+## Concepts encountered while solving it
+
+### Code formatting
+
+It's the first time most students work with Go, and many have not yet used `gofmt`.
+
+### Linting
+
+It's the first time most students work with Go, and many have not yet used `golint`. Linting already provides some hints
+on what (not) to do, e.g. use of variable names, not using else if the the regular condition ends with return, and missing
+documentation on exported function (or in the wrong format). 
+
+### Comments
+
+It's the first time most students work with Go, and many have not yet been using proper documentation of the package and
+the exported function. Most of the time they leave the stub comments in the file.
+
+### Functions
+
+Everything is done via functions. In this case:
+
+- there is no concept of default values for parameters; it's done in the body of the function
+- parameters are passed by value, so in the scope of the function they can be altered
+
+### Strings
+
+The parameter passed is a string. For the default value, a check needs to be made if the string is empty.
+
+- how to do an idiomatic check for empty string
+- how to concatenate strings, either using a simple `+`, or the `fmt` package
+
+
+### Variables / Assignment / Scope
+
+The parameters are passed by value, so in the scope of the function they can be altered.
+
+- scoping
+
+### Comparison / Conditionals
+
+The code needs to check whether the parameter is empty.
+
+No else branch is needed. Keeps the code simpler.
+
+
+### Return value
+
+Value needs to be returned. No naked return.