Add contribute.md and changelog

[nuest]

For the reasoning, see http://contribute.md/

I'd create a first version (partially based on https://github.com/openlayers/ol3/blob/master/CONTRIBUTING.md), but I don't want to imply to much regarding community process etc. without a discussion, so here is a draft: https://gist.github.com/nuest/4b6d60ec0d8fd3b6a930

Feedback is highly appreciated.

Questions:

• How does QUnit work - will all tests go into index.html, or will we split these up by service? Do we have end to end tests (yet)?
• Will we have a manual changelog, or rely on pull request history? I'm inclined for outward communication to have a changelog.md (or a preferred format for JavaScript libs). This looks like a good example: https://github.com/angular/angular.js/blob/master/CHANGELOG.md
• Code style and guidelines - should we set these up now? https://github.com/openlayers/ol3/blob/master/CONTRIBUTING.md gives some extensive and precise guidelines.

I surely don't want to slow down development by this, so keeping these things low profile is fine by me, then we'll just mention that in the contribute.md 😄

[tomkralidis]

@nuest thanks for kicking this off. Comments:

• QUnit testing is still TBA (suggestions welcome!); having said this I would envision tests split up by service. We have some preliminary tests for CSW but nothing concrete
• -0 on changelog file given commit history (perhaps we can automate changelog create/update?)
• there are some jshint settings in https://github.com/OSGeo/ows.js/blob/master/.jshintrc but by no means final and open to suggestion