Filled out credits for everyone who contributed

Over the last eight years, the list has grown pretty large.  Makes me
feel good.

I looked through CHANGES file to get the list.  If I forgot you, please
do not take this personally.  Email me and I'll fix it (or open a pull
request).

I did not list some co-maintainers.  This is because I could not find
code that they contributed.  Perhaps their contributions are still in
the future.