Add info for reviewers of outside developer commits.

Add the start of a section that talks about the responsibilities of
Chapel core team reviewers of outside developers' commits.

While here, fix a typo in a cross-ref.
I think I would refer to "Chapel project leadership" rather than "Chapel manager" above, primarily because the phrase seems more general for the open-source world (where this lives) and works in a Chapel Foundation setting as well (where "manager" seems very specific to the Cray team).  I'd also drop the final sentence, not because it's not true, but to avoid a case where an external contributor uses it as justification for not needing to sign the contributor agreement.  Then, I'd transition into the next paragraph by starting it out with something like "Care may need to be taken when committing third-party code that originates from a different git[hub] repository."  And then go into your "As an example..." text.  At which point, I think this is OK to commit.