Add README.md

[dandv]

Package is now popular enough to be used as an example by other packages. A README would be great.

[mrmnmly]

+1 I'm also looking forward for it because I'm looking for file-uploading/handling tool and found this one, but I have no idea how to use it properly (I've never before implemented file uploaders)

[mkubera]

+1 I'm in the same situation as ofcapl. An explanation of how to use it would be grand :)