Make working with git easier

Created by: fxcoudert

1. Git does not preserve empty folders, so upon checkout the bin/ and obj/ folders do not exist. Compilation then fails because the directories do not exist. Convention to preserve these directories is to create a .gitkeep file in them.

2. After compilation, git indicates that some “untracked files” are present: obj/*.o, *.mod and bin/atomes. We can tell it that those files are outside source control, and it will ignore them.