Update Developer Guide

[ruflin]

The developer guide needs an update that includes https://github.com/elastic/beat-generator

• Document beat-generator and how to use it
• Document on how to add a vendor library (remove .git folder)
• Document recommendations for .editorconfig (how to use it)
• Document on how to use travis
• Document how fields.yml is used

[monicasarbu]

I would include the beat-generator documentation under the Developer Guide: Creating a New Beat section. As of #1172, we need to update the developer guide anyway to replace Topbeat with some other simple beat.

[ruflin]

Agree. For the guide I was thinking to take only the beat that is generated by beat-generator.

[monicasarbu]

I think we should add a link to David's blog post about soundbeat as it describes the full experience of a Beat creator: http://david.pilato.fr/blog/2016/03/17/and-the-beats-go-on/

[ruflin]

Closing this issue as @kimjmin did a blog post (https://www.elastic.co/blog/build-your-own-beat) and the dev guide got updated accordingly.