scripts/checkpatch.pl: fix README.md file name after rename

scripts/checkpatch.pl: fix README.md file name after rename

checkpatch.pl uses a list of files to detect the root OpenWrt
directory. This includes README, which has been renamed to
README.md in the previous commit.

Update the file name in checkpatch.pl to prevent errors like the
following when running the script:

   Must be run from the top-level dir. of a OpenWrt tree

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>