backwards-compatibility: fix broken link
The breaking-changes-guide has moved to bazelbuild/bazel-website
which is served from bazel.build, rather than docs.bazel.build.
So we cannot use a relative link. Fix this.
@@ -28,7 +28,7 @@ incompatible and require some changes from Bazel users.
### How to follow this policy
-* [For contributors - best practices for incompatible changes](../../breaking-changes-guide.html)
+* [For contributors - best practices for incompatible changes](https://bazel.build/breaking-changes-guide.html)
* <a href='https://github.com/bazelbuild/continuous-integration/tree/master/docs/release-playbook.%6D%64'>For release managers - how to update issue labels and release</a>
