Add old README file back for bundling in packages. README.md can be developer oriented.
Update README.md to be more developer oriented.
Fix links and formatting.
Fix links, punctuation.
Social media.
Remove reference to 'mixxx' directory. Add libchromaprint to dependency list.
Use new Facebook page link.
Fix version/branch reporting.

* Include build.h in dlgabout.cpp
* Call MixxxApp::logBuildDetails() in MixxxApp constructor.
update git ignore to exclude .DS_Store and Xcode project folder