Add rust-mac branch build status into README.md.
Update README.md contents.

 * wxWidgets 3.0 is now released and recommended.
 * `brew install wxmac` installs wx 3.0 now. `--devel` flag is not required.
 * USE_MACPORTS cmake option is deleted (MacPorts build doesn’t need that).
Merge branch 'rust-servo' into rust-mac: README: Add rust-mac branch build status and update contents.