add CONTRIBUTING.md

this contains some of the basics for contributing patches to the
project. over time the instructions from
https://www.webmproject.org/code/contribute/submitting-patches/ can be
added here.

Change-Id: Ib5fcbc818c6feb87654517fdd7118f226c5cccc3

CONTRIBUTING.md

@@ -0,0 +1,29 @@
+# How to Contribute
+
+We'd love to accept your patches and contributions to this project. There are
+just a few small guidelines you need to follow.
+
+## Contributor License Agreement
+
+Contributions to this project must be accompanied by a Contributor License
+Agreement. You (or your employer) retain the copyright to your contribution;
+this simply gives us permission to use and redistribute your contributions as
+part of the project. Head over to <https://cla.developers.google.com/> to see
+your current agreements on file or to sign a new one.
+
+You generally only need to submit a CLA once, so if you've already submitted one
+(even if it was for a different project), you probably don't need to do it
+again.
+
+## Code reviews
+
+All submissions, including submissions by project members, require review. We
+use a [Gerrit](https://www.gerritcodereview.com) instance hosted at
+https://chromium-review.googlesource.com for this purpose. See the
+[WebM Project page](https://www.webmproject.org/code/contribute/submitting-patches/)
+for additional details.
+
+## Community Guidelines
+
+This project follows
+[Google's Open Source Community Guidelines](https://opensource.google.com/conduct/).