Add contributer's guide

[beatgammit]

Now that #179 has been merged, now is a good time to make a contributer's guide. I think the main points should be:

• run through dartfmt, using 120 for max line length
• prefer Dart conventions over a direct port from three.js
• make sure tests pass on your machine before submitting a pull request

Any other ideas?

[nelsonsilva]

I was just that thinking dartfmt and dartanalyzer should be ran as part of the build process which could be triggered by pull requests.