docs: update credtis, point to the repository log

The docs/CREDITS file dates back to the SVN repository.
Now that we use Mercurial, the repository stores appropriate
authorship for each commit. Say so in the CREDITS.

 reporter_name="Yann E. MORIN"

 reporter_url="http://ymorin.is-a-geek.org/projects/crosstool"

 reporter_comment="Experimental toolchain for ARM cortex-m3 in Thumb-only, noMMU, and newlib."