Some minor README.md updates.
Some minor README.md updates.

- Fix a typo.
- Add the mingw toolchain files.

Change-Id: I3a3b8157457500757059a27dcc10c63a452760f7

GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom