Added plugins docs page
There is now a page docs/source/plugins.rst which will house all of the docs
for orvsd_central's plugins. Currently the file only has information on the
branching scheme of the plugins.
Fixed formatting of codeblock in plugins.rst
Changed plugins.rst to comment on moodle-plugins
Now the docs explicitlt state that plugins are for moodle and not
orvsd_central.
There was also a paragraph added to the beginning of plugins.rst
which comments on plugins and how they relate to the project. This
paragraph can be expanded later for further clarification.
Added guidelines for how to branch moving forward.

Added how to branch moving forward (eg 2.9_develop is branched off of
2.7 branch, 2.9 branch is off of 2.9_develop).

Clarified that master does not serve a purpose since we changed our
workflow.
Merge pull request #73 from orvsd/docs_plugins

Added plugins docs page