Make CONTRIBUTING.md all caps as per the Charter
Make CONTRIBUTING.md all caps as per the Charter

According to the official Charter at
https://www.openbmc.org/wp-content/uploads/sites/62/2018/03/charter_openbmc_02142018.pdf
the file must be named literally `CONTRIBUTING`.
That is, all caps.

This has been discussed in review comments for
https://gerrit.openbmc-project.xyz/9948

This commit accompanies the corresponding changes to
the docs repository, see
https://gerrit.openbmc-project.xyz/c/openbmc/docs/+/10807

Change-Id: Ia226289caa5c0d52875c011a9f31ffdf52cf30e3
Signed-off-by: Alexander Amelkin <41b32ffda89ef4e82ad516c4fce0571445749fe0@yadro.com>

openbmc/openbmc-test-automation,openbmc/openbmc-test-automation