Documentation and Makefile tweaks

[jbcpollak]

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.

[ryansb]

Tested locally, the changes work for me.