Add Docs About Files in Repo

[wbond]

Add some documentation about how every file in a repo is downloaded by every user. Thus package developers shouldn't host a bunch of images in their repo. We should do some testing to see if they can reference an image from one branch in the readme of another.

[FichteFoll]

You can use direct links to the raw files I guess. However, I'm not sure about relative urls and if they actually work as ![img][../img/image.png] images so I'll test that soonish.

Edit: Actually, I cba to test because it will most likely not work anyway. So, just let them use absolute urls to the raw files (on a different branch) or host on an external hoster such as imgur.com. Hell, they could even upload the image to github via the comment box and just use the url from there.

[FichteFoll]

FYI, you can tell github to not include certain files in its source archives (from branch, commit or tag) with .gitattributes. See http://git-scm.com/docs/gitattributes and https://github.com/gerardroche/sublime-php-grammar/blob/master/.gitattributes for an example.