BaseTools: rename ReadMe.txt to ReadMe.rst
The ReadMe file is now RST, so update the filename extension to match.
Signed-off-by: Rebecca Cran <rebecca@bsdio.com>
Cc: Bob Feng <bob.c.feng@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Reviewed-by: Bob Feng <bob.c.feng@intel.com>