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.
Merge pull request #130 from gberaudo/olcs_core_angles

Olcs core angles
Introduce olcsx namespace for the exported typedefs from the library.

All typedefs marked `@api` must be moved here to prevent closure minification.
Merge pull request #131 from gberaudo/introduce_olcsx

Introduce olcsx
Add an olcs.getOlMap() getter
Merge pull request #132 from gberaudo/olmap_getter

Olmap getter
Core function for applying an height offset to an ol3 geometry
Merge pull request #133 from gberaudo/geometry_offset_core

Geometry offset core
Core Cesium function for animated rotation around axis

May be used by application developers to implement:
- 2D/3D transition (tilt);
- orientation transition (heading).
Merge pull request #134 from gberaudo/animated_rotations

Animated rotations
Add CesiumMath.toRadians to externs
Add Cartesian3.clone to extern.
Add custom constraint axis angle in Cesium externs.