Documentation and Makefile tweaks

Hi Ben,

Based on issue https://github.com/benblazak/ergodox-firmware/issues/46, here is a small update that does a few things to help get newbies started:

1) I clarified the compilation documentation to help new users get started. You don't need to compile from the src directory with the makefile changes below...
2) use -f with git clean, this prevents an error when running `make clean`
3) tell git to ignore the build directory
4) the `firmware` target will put the firmware in the `build` directory (`firmware-compile` does what `firmware` used to do)
5) added a `firmware-all` target that will skip building the ui-info files (just runs `firmware` for all layouts)

I hope you approve of these changes, but let me know what you think.