Rewrite of contribution guide per C4

GeoMet now uses a modified version of the C4 process. CONTRIBUTORS.txt
has been renamed to AUTHORS.txt.
Merge pull request #11 from larsbutler/contributing

Rewrite of contribution guide per C4
README: s/larsbutler/geomet/g
Merge pull request #12 from larsbutler/README-cleanup

README: s/larsbutler/geomet/g