Reorganize docs and add contributing guides

This change introduces a reorganization of the information in the
README.md document, which has been split into several files in the
//docs folder, and adds the following information:

- //docs/contributing.md: New document containing information
  about how to contribute to the project.
- //docs/developers/build.md: Add information about CHROMIUM_MV3
- //docs/developers/set_up.md: New document containing information on
  how to set up the development environment.
- //docs/developers/add_feature.md: Blank document which will contain
  info on how to add a new feature/option to the extension.

Fixes: #26
Change-Id: I44c69ccbc20e609c5416e08fe0ef3b7934080d17
diff --git a/docs/developers/set_up.md b/docs/developers/set_up.md
new file mode 100644
index 0000000..650ce31
--- /dev/null
+++ b/docs/developers/set_up.md
@@ -0,0 +1,15 @@
+# Set up the development environment
+Before starting to develop, you must set up your device. This is what you must
+do:
+
+1. [Install Go](https://golang.org/doc/install). On Mac, you can install it
+with [Homebrew](https://brew.sh/) by running `brew install go`.
+   - This is because the build script uses a Go program to generate the
+   manifest.
+2. That's it! If you're using a Mac, you're out of luck, because you must
+perform some more steps.
+
+## Mac-specific configuration
+On a Mac, you must also follow these steps:
+
+TODO(Issue #18): add macOS steps