Add installing addons page to getting started.

I could only find the old [Installing a third party product](http://developer.plone.org/reference_manuals/old/buildout/thirdparty.html) page so a wrote an updated / shorter one.

- I think `mr.developer` should be used and packages should not be cloned manually and added to `develop` in buildout.
- I didn't cover old school Products, since most up to date add-ons are distributed as egg.
- I dropped all the ZCML stuff, since most of the add-ons have a `z3c.autoinclude` entry point nowadays.

I recently started the [plonesource.org](http://plonesource.org) project (sources for mr.developer) and was threatened to be thrown in a cold swimming pool unless I document plonesource.org here :open_mouth:
That's why I've used plonesource.org in the `sources.cfg` example, although I'm not sure whether it belongs there or should be moved to the `external` or `reference_manuals` parts. Feel free to place your opinion and I'm happy to update my pull-request :wink: 

When the PR is merged I'll also update plonesource.org and add a link to the developer manual.