Add contribution guidelines file

Add initial version of `contributing.md`.

GitHub would show a link to this file on the new issue page and when opening a
PR. 
https://help.github.com/articles/how-do-i-set-up-guidelines-for-contributors

Remove .gitignore as there is nothing to ignore from this project. GitHub
recommendation is to have just project-specific paths ignored in the
.gitignore file of the repo. Platform-specific files and editor-specific files
should go in a global gitignore file. 
https://help.github.com/articles/ignoring-files#global-gitignore

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I guess you could always write your own guidelines or edit these ones, but it's a start.

I could split it up in separate PRs or remove some parts of this one if you'd like.