Bazel docs: make it more clear that we can't yet accept pull requests.

The contributions.md file used to say this at the bottom, where this info
is out of view. I've moved it up where it's more visible.

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MOS_MIGRATED_REVID=89495841

docs/contributing.md

@@ -20,12 +20,13 @@ and work on the Bazel code.
 6. The patch is exported as a Git commit, at which point the Gerrit code review
    is closed.
 
-We do not currently accept pull requests on GitHub.
+**We do not currently accept pull requests on GitHub.**
 
 We will make changes to this process as necessary, and we're hoping to move
 closer to a fully open development model in the future (also see
 [Is Bazel developed fully in the open?](governance.md#isbazelopen)).
 
+
 ## Setting-up your coding environment
 
 For now we only have partial support for the Eclipse IDE for Java. We don't have