Add a DEVELOPMENT.md

[webmat]

This would be helpful to help a new contributor get started right away without too much fiddling around

Technical bits:

• Python version 2.7 or 3+ compatible
• Version of Go (see Beats?)
• make setup to install Python deps
• make to generate the automated parts
  • Don't edit readme, edit the parts under docs/

Process bits:

• Pull requests / who merges, etc
• Signing the CLA

What else?

I'm all for doing this myself. But I'd like your input on what I'm forgetting @ruflin @andrewkroh :-)

[ruflin]

What about the CONTRIBUTING.md here? https://github.com/elastic/ecs/blob/master/CONTRIBUTING.md

[webmat]

Not sure how I missed this file LOL.

Let's keep the issue open for now. I'll add some ideas to CONTRIBUTING.md, as not all of my points above are addressed by the file.