Add a CONTRIBUTING.md file.

Change-Id: I4e1ed0aaddf4dc516a81155ef62dba138f8495ae
Reviewed-on: https://boringssl-review.googlesource.com/7120
Reviewed-by: Adam Langley <agl@google.com>
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
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+Want to contribute? Great! First, read this page (including the small print at the end).
+
+### Before you contribute
+Before we can use your code, you must sign the
+[Google Individual Contributor License Agreement](https://cla.developers.google.com/about/google-individual)
+(CLA), which you can do online. The CLA is necessary mainly because you own the
+copyright to your changes, even after your contribution becomes part of our
+codebase, so we need your permission to use and distribute your code. We also
+need to be sure of various other things—for instance that you'll tell us if you
+know that your code infringes on other people's patents. You don't have to sign
+the CLA until after you've submitted your code for review and a member has
+approved it, but you must do it before we can put your code into our codebase.
+Before you start working on a larger contribution, you should get in touch with
+us first via email with your idea so that we can help out and possibly guide
+you. Coordinating up front makes it much easier to avoid frustration later on.
+
+### Code reviews
+All submissions, including submissions by project members, require review. We
+use [Gerrit](https://boringssl-review.googlesource.com) for this purpose.
+
+#### Setup
+If you have not done so on this machine, you will need to set up a password for
+Gerrit. Sign in with a Google account, visit
+[this link](https://boringssl.googlesource.com/), and click the "Generate
+Password" link in the top right. You will also need to prepare your checkout to
+[add Change-Ids](https://gerrit-review.googlesource.com/Documentation/cmd-hook-commit-msg.html)
+on commit. Run:
+
+    curl -Lo .git/hooks/commit-msg https://boringssl-review.googlesource.com/tools/hooks/commit-msg
+    chmod u+x .git/hooks/commit-msg
+
+#### Uploading changes
+To upload a change, push it to the special `refs/for/master` target:
+
+    git push origin HEAD:refs/for/master
+
+The output will then give you a link to the change. Add `agl@google.com` and
+`davidben@google.com` as reviewers.
+
+Pushing a commit with the same Change-Id as an existing change will upload a new
+version of it. (Use the `git rebase` or `git commit --amend` commands.)
+
+For more detailed instructions, see the
+[Gerrit User Guide](https://gerrit-review.googlesource.com/Documentation/intro-user.html).
+
+### The small print
+Contributions made by corporations are covered by a different agreement than
+the one above, the
+[Software Grant and Corporate Contributor License Agreement](https://cla.developers.google.com/about/google-corporate).