Copy CONTRIBUTING.md from AOSP.

CONTRIBUTING.md

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+Contributing to the Android NDK
+===============================
+
+This guide will help you contribute to the Android NDK, whether by reporting
+issue or submitting a patch.
+
+Reporting Issues
+----------------
+
+Our bug tracker is hosted on GitHub: https://github.com/android-ndk/ndk/issues.
+
+**Note:** Bugs against Android Studio or Gradle should be filed at
+http://b.android.com, not here.
+
+Before filing a bug, check that you are using the latest version of the NDK. The
+latest stable version and the preview release of the next stable version are
+listed on our wiki: https://github.com/android-ndk/ndk/wiki#downloads.
+
+Submitting Patches
+------------------
+
+While we use GitHub for our bug tracking and documentation, we do not manage our
+code in GitHub. Like the rest of Android, we are hosted in AOSP. This means that
+patches are uploaded to and submitted via Gerrit.
+
+Follow the [contribution instructions for
+AOSP](http://source.android.com/source/submit-patches.html). When you run `repo
+init`, follow the instructions in the NDK's [README.md]. We have a different set
+of dependencies than AOSP, also noted in our [README.md].
+
+Instructions for building the NDK and running our tests are documented in
+[README.md] and [Testing.md](docs/Testing.md). Note that the NDK cannot be built
+on Windows; only Linux and Darwin hosts are supported.
+
+Check the [OWNERS file](OWNERS) for reviewers.
+
+[README.md]: README.md