mingw: README.txt -> README.md.
mingw: README.txt -> README.md.

The file in the parent directory was renamed; change the
reference in the Makefile as well.

xiph/opusfile,xiph/opusfile,xiph/opusfile,SmartWalkieOrg/opusfile,SmartWalkieOrg/opusfile,SmartWalkieOrg/opusfile