README: Add reference link to rust-sdl2_mixer
Merge pull request #72 from lifthrasiir/i686-compat-rwops

Make `RWops` compatible to i686 platform; Also add more patterns to .gitignore.
Merge pull request #74 from andelf/add-ref-link-to-rust-sdl2_mixer

README: Add reference link to rust-sdl2_mixer
Merge remote-tracking branch 'upstream/master'