Sdk: Create a Dart Code of Conduct.

[pq]

Follow-up from https://github.com/dart-lang/linter/pull/718.

The gist is to add a simple code of conduct in the spirit of other inclusive OSS projects (and the advice of github).

As per @anders-sandholm's suggestion, this file would live in the root of the SDK as well standard template repo so it is automatically included in new repos going forward. Existing projects like the linter can manually sync up.

[kwalrath]

We'll be doing this on www.dartlang.org. See dart-lang/site-www#539.

[pq]

This is fixed, and live here: https://www.dartlang.org/code-of-conduct

Thank you!