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
Merge pull request #136 from gberaudo/contributing.md

Basic contributor readme
Add core functions to pick canvas bottom and center points.
Camera cleanup.
Merge pull request #128 from gberaudo/camera_cleanup

Camera cleanup
Warm up 3D function.

By pre-loading Cesium, it allows a faster switch to 3D.
The camera will be positioned using the provided height
and the 2D coordinates from the ol3 view.
Merge pull request #129 from gberaudo/olcs_warmup

Olcs warmup
Add core functions to compute angles.