Issue-#20 Add file with contribution guidelines

Here's a draft of the contribution guidelines for mod-lang-scala. Please have a read and comment accordingly. 

Don't integrate this pull request yet cos I wanna send it alongside the project admin guidelines document too, so that all guidelines are in place. I'm just posting it here for the moment to get feedback.

I'll add the section of Git tips alongside the project admin guidelines (issue #32) for which I'll be sending a commit in the next few days.