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.
Add a note about ugni memory registration and concurrency with slurm.
Merge pull request #587 from gbtitus/doc-slurm-ugni-mem-registration-issue

Add a note about ugni memory registration and concurrency with slurm.

Reviewed by Thomas.
Fix nondeterministic output from walkdir() by requesting sorted output
Merge pull request #590 from bradcray/fix-filerator-regression

Fix nondeterministic output from walkdir() by requesting sorted output

[trivial, not reviewed]

I forgot to request sorted output in my new test yesterday, which
resulted in nondeterministic behavior across file systems/machines.
This broken in one configuration last night (and could break in others
over time).
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.