Docs pages

[pp-mo]

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

[pp-mo]

Done #76