Update README.md contents.
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).

tempbottle/wxRust,tempbottle/wxRust