Add DEVELOPING.md
Add DEVELOPING.md

I started writing some developer doc.  For now, it explains how to run
the examples from scratch.  I expect to add more, like how the watchers
work, and how one can efficiently develop and test the different
components.

Signed-off-by: Simon Marchi <fa7054fed17d58b06d3b1bf9b1077e42165a09ec@ericsson.com>

lmcgupe/theia,lmcgupe/theiagreenkeeperTest,lmcgupe/theiagreenkeeperTest,epatpol/theia-1,hexa00/theia,hexa00/theia,epatpol/theia-1,lmcgupe/theia,lmcgupe/theiagreenkeeperTest,epatpol/theia-1