Contribution guidelines draft

The first draft of the contribution guidelines, #1.
> Files from your personal IDE/Editor/system that should be ignored go in .git/info/exclude.

I don't personally use the `.git/info/exclude` file, what I use is the global settings at `~/.gitignore_global` but I didn't think it was necessary to put the specific instructions on how to do this in the guidelines.

> Files that ALL people on the project will want to ignore" go in .gitignore.

Even if all of us use vim doesn't mean we should put `.swp` files in the project's `.gitignore`, as the `.swp` entry has nothing to do with the project. I don't think there is anything wrong with the current wording because even if something is used by all it can still be a personal setting that has nothing to do with the project.