ReadMe: Convert to rst and add Platform CI Status

REF:https://bugzilla.tianocore.org/show_bug.cgi?id=2570



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>