Scala-native: Developer guide

[densh]

Things that need to be covered in the guide:

• [x] Building and running Scala Native projects with sbt
• [x] Overview of low-level language extensions in Scala Native
• [x] Writing bindings for C libraries
• [x] Publishing NIR for Scala libraries with sbt

[frgomes]

There's a simple proof of concept, whic is a separate example application at:
http://github.com/frgomes/poc-scala-native

The README contains hints for building on Linux.
I'm trying to keep it up-to date with recent developments in master.

[densh]

Need to mention that Java 8 is required as a build dependency (to avoid https://github.com/scala-native/scala-native/issues/179)

[densh]

I've been working on this lately. We have a new docs website at scala-native.readthedocs.io. There's not much text on the language and interop yet, that's something that still remains to be covered.