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CONTRIBUTING.md

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+# Guidance
+
+> Contributing to an open-source codebase for the first time can be intimidating --
+huge files with functions that call other functions in other huge files.
+>
+> This doc aims to make contributing a little friendlier.
+
+Given the nature of this project, you'll most likely want to start at [`docs.ex`](https://github.com/elixir-lang/ex_doc/blob/master/lib/mix/tasks/docs.ex#L230)
+
+## docs.ex
+
+> If you're unfamiliar with [`Mix.Task`s](https://hexdocs.pm/mix/Mix.Task.html)...
+
+1. When a user runs `mix docs`, `run/3` is executed. As noted in the `@moduledoc`,
+the `args` are from the command line. As we'll be generating docs from the task,
+the first step is to compile our user's Mix project.
+
+2. We guard against bad command line `args`
+
+3. After setting up `options`, we generate docs in a number of formats and
+log messages with where our user can find them in the project.
+
+## ex_doc.ex
+
+So what's this [`generator/3`](https://github.com/elixir-lang/ex_doc/blob/master/lib/mix/tasks/docs.ex#L264) anonymous function being called? Well, it [defaults](https://github.com/elixir-lang/ex_doc/blob/master/lib/mix/tasks/docs.ex#L240) to `ExDoc.generate_docs/3`, so let's open up [`ex_doc.ex`](https://github.com/elixir-lang/ex_doc/blob/master/lib/ex_doc.ex#L18) to see what that function does...
+
+4. `generate_docs/3` has guards to ensure arguments are a certain type.
+5. We generate a `config`. This will be a [big struct](https://github.com/elixir-lang/ex_doc/blob/master/lib/ex_doc/config.ex#L18) which we feed `options` into.
+6. But first, we once again need to setup our `options`.
+7. Next, back in `generate_docs/3`, we call [two `ExDoc.Markdown` functions](https://github.com/elixir-lang/ex_doc/blob/master/lib/ex_doc/markdown.ex#L136) with side-effects.
+
+## retriever.ex
+
+8. 
+
+# Guidelines
+
+* TODO