user-manual: Add section on ignoring files

user-manual: Add section on ignoring files

The todo list at the end of the user manual says that something must be
said about .gitignore. Also, there seems to be a lack of documentation
on how to choose between the various types of ignore files (.gitignore
vs. .git/info/exclude, etc.).

This patch adds a section on ignoring files which try to introduce how
to tell git about ignored files, and how the different strategies
complement eachother.

The syntax of exclude patterns is explained in a simplified manner, with
a reference to git-ls-files(1) which already contains a more thorough
explanation.

Signed-off-by: Johan Herland <johan@herland.net>