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/add_feature.md b/docs/developers/add_feature.md
new file mode 100644
index 0000000..c8e81b2
--- /dev/null
+++ b/docs/developers/add_feature.md
@@ -0,0 +1,2 @@
+# Add a new feature
+TODO(Issue #28): write this doc