Basic contributor readme
Basic contributor readme

Adaptation of the OpenLayers3 CONTRIBUTING.md.
Rules for checking for `undefined` and `null` as discussed at
https://github.com/openlayers/ol3-cesium/pull/128

fredj/ol3-cesium,camptocamp/ol3-cesium,GistdaDev/ol3-cesium,openlayers/ol3-cesium,thhomas/ol3-cesium,GistdaDev/ol3-cesium,openlayers/ol3-cesium,fredj/ol3-cesium,thhomas/ol3-cesium,gejgalis/ol3-cesium,openlayers/ol-cesium,oterral/ol3-cesium,epointal/ol3-cesium,fredj/ol3-cesium,oterral/ol3-cesium,epointal/ol3-cesium,bartvde/ol3-cesium,thhomas/ol3-cesium,gejgalis/ol3-cesium,gberaudo/ol3-cesium,openlayers/ol-cesium,bartvde/ol3-cesium,gberaudo/ol3-cesium,gejgalis/ol3-cesium,camptocamp/ol3-cesium