Make documentation (more) visible

[me-kell]

The documentation is distributed in four .rst files and concatenated to the variable long_description
in setup.py (see below).

To make it easier to find the documentation put at least a link to those files. Better; generate docs.

long_description = (
    read('README.rst') +
    '\n' +
    read('plone', 'registry', 'registry.rst') +
    '\n' +
    read('plone', 'registry', 'events.rst') +
    '\n' +
    read('plone', 'registry', 'field.rst') +
    '\n' +
    read('CHANGES.rst') +
    '\n'
)