Document extension entry points

[Stiivi]

Document how to add entry_points into setup.py to advertise Cubes backends.

The setup.py of extension module should contain something like this:

entry_points = {
    'stores': ['mystore = mystore.store:MyStore'],
    'browsers': ['mybrowser = mystore.browser:MyBrowser'],
}

Example problem: #350

References:

• cubes.ext module
• pkg_resource documentation

[Stiivi]

Example of server entry_points configuration for the Flask slicer server.

[Stiivi]

Use description of PR #427 as a starting point for documentation.

[Stiivi]

See #394 and #430.

[Stiivi]

Closing this one, as the extension points mentioned in the original issue are obsolete.

[Stiivi]

Replaced by #434