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.
Minor update to reflect review feedback.
Merge pull request #522 from gbtitus/update-contrib-practice

Add info for reviewers of outside developer commits.

Add 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.

Reviewed by Brad.