Readme: split contributing section into its own file

A contributing file is a de facto standard today.
In particular, GitHub even uses it to display the information about
contributing in appropriate locations:
https://github.com/blog/1184-contributing-guidelines
Contributing: remove the suggestion to contact @imfilp on Twitter

@filp is now less actively involved with the project.
People contacting him on Twitter directly will both bother him,
and delay the useful feedback getting back to the project.