Docs pages

Provide summary user documentation

Probably with Sphinx, since that is good for #29 ?

Becomes desirable as soon as too unwieldy for current simple single-page README approach.
When we want to explain specific details, such as ...
  * API docs : #29 
  * unsupported features of netcdf4 (user types) : #28 
  * handling of string + character data : #28 
  * separate release-notes : #31 