Convert CONTRIBUTING.rst and README.rst to MarkDown

Summary

I've been converting all the parent text files to MarkDown in an effort to make things friendly to GitLab for the %6.1 release.

libbsd is the only repository to actively move to reST and while we do use this for our documentation it's out of necessity due to the complexity required to have good documentation. For source docs we should stick with MarkDown. I've already moved every other RTEMS repository this is the last one outside of rtems-central.

Is there a reason you've moved these to reST are you OK with me moving them to MarkDown?

This question is for @sebhub.

Thank you.