We need a good readme for documentors.

[timlinux]

Problem

When arriving at the inasafe-doc repo, there is no obvious starting point

Proposed solution

On the QGIS-Documentation page we have a really nice 'getting started' readme for new document writers and translators. We should adapt this content to produce something similar for ourselves. See https://github.com/qgis/QGIS-Documentation

Expected outcome

New document writers will find the help the need to get started when they arrive at https://github.com/AIFDR/inasafe-doc