Add quick Architecture doc + code flow diagram

[ci skip]

Author: mcmire

ARCHITECTURE.md

@@ -0,0 +1,22 @@
+# Architecture
+
+I'll have more later around this,
+but here are some quick hits:
+
+## Basic concepts
+
+* A **differ** figures out which operational sequencer to use.
+* An **operational sequencer** makes a comparison between two like data structures.
+  and generates a set of operations between them
+  (additions, deletions, or changes).
+* An **operation sequence** is a list of those operations
+  associated with some kind of diff formatter.
+* A **diff formatter** takes a list of operations
+  and spits out a textual representation of that list in the form of a conventional diff.
+* An **object inspector** generates a multi-line textual representation of an object,
+  similar to PrettyPrinter in Ruby or AwesomePrint,
+  but more appropriate for showing within a diff.
+
+## Code flow diagram
+
+[![code flow](./docs/code-flow-diagram.png)](https://docs.google.com/drawings/d/1nKi4YKXgzzIIM-eY0P4uwjkglmuwlf8nTRFne8QZhBg/edit)

CONTRIBUTING.md

@@ -35,3 +35,9 @@ I'll try to respond as quickly as I can.
 I may have suggestions about code style or your approach,
 but hopefully everything looks good and your changes get merged!
 Now you're a contributor! 🎉
+
+## Understanding the codebase
+
+If you want to make a change
+but you're having trouble where to start,
+you might find the [Architecture](./ARCHITECTURE.md) document helpful.

README.md

@@ -85,7 +85,7 @@ expect(actual).to eq(expected)
 
 You would get output that looks like this:
 
-![Before super_diff](doc/before.png)
+![Before super_diff](docs/before.png)
 
 What this library does
 is to provide a diff engine
@@ -94,7 +94,7 @@ and display them in a sensible way.
 So, using the example above,
 you'd get this instead:
 
-![After super_diff](doc/after.png)
+![After super_diff](docs/after.png)
 
 ## Installation