The private folder is for convenience, for putting some files away and
for own TODO lists; generally for data which is related to the project
but shouldn't be committed.
The build folder contains intermediate/object files for building the
different stages; it should be ignored explicitly, even though the
file ending of all files marks the folder as ignored implicitly.
This commit adds a basic .gitignore which only contains machine code
file endings; files which shouldn't be committed as they're better to
be distributed in other ways.