Convert workspace root Readme.md to reStructuredText
ReStructuredText supports building tables with syntax that is easier
to read in plain-text.
Rename as ReadMe.rst.
Add Platform CI Status Tables
Cc: Andrew Fish <afish@apple.com> Cc: Laszlo Ersek <lersek@redhat.com> Cc: Leif Lindholm <leif@nuviainc.com> Cc: Michael D Kinney <michael.d.kinney@intel.com> Signed-off-by: Michael Kubacki <michael.kubacki@microsoft.com> Reviewed-by: Laszlo Ersek <lersek@redhat.com> Reviewed-by: Bret Barkelew <bret.barkelew@microsoft.com> Reviewed-by: Leif Lindholm <leif@nuviainc.com>