CONTRIBUTE: More formally specify the commit description

[dfandrich]

This codifies what people have actually used in git commits over the
past 6 years. I've left off some lesser-used headers that appear to
duplicate others and tried to describe a consistent use for several
others that were used more arbitrarily.

This makes it easier for new committers to find out the kinds of things
we want to acknowledge, makes it easier to perform statistical analysis
on commits, and opens the possibility of performing lint checks on
descriptions before submission.

QUESTIONS:

The number of endorsed header types could be reduced and simplified, e.g.:

• Use Reported-by: instead of Found-by:
• Use Assisted-by: instead of Tested-by:
• Use Acked-by: instead of Approved-by:
• or use Reviewed-by: instead of Acked-by: and Approved-by:
• Use Authored-by: for each author instead of Co-Authored-by:

Endorsing them all does allow slightly more fine-grained information at the
price of slightly higher cognitive load reading them.

[jay]

I don't think we need every instance of -by to be explained, we already know and state whatever-else-by: is an attribution. Some of the more common terms could be suggested on a single line like "Some common attribution terms are Reviewed-by, Signed-off-by, etc". Acked-by and tested-by aren't used like ever AFAICT.

[dfandrich]

Jay's suggestion makes the text considerably smaller and more approachable so I've done that.