Add developer documentation

As the SSR is quite complex it is not easy for a new developer to get started, see for example this blog post http://people.bath.ac.uk/djf32/ssr.html.

We should add an "Introduction for developers" at some point in the documentation. We need to decide where this is best suited.

Good to know for new developers

The main page is our web site at http://spatialaudio.net/ssr/ which is hosted using Wordpress.
The manual is available as HTML (http://ssr.rtfd.io/) and PDF (https://media.readthedocs.org/pdf/ssr/latest/ssr.pdf)
The manual is automatically build by ReadTheDocs by every push of the master branch from the directory doc/manual/
The manual can be build locally by executing make html inside doc/manual/
In our wiki we plan upcoming features.
An API documentation can be build by rnunning doxygen by executing make doc in the main dir.
There is a deep connection between the SSR and APF. APF is integrated via git subtree at apf/. Have also a look at the corresponding explanation how this was done.
An API documentation for APF is available and is build manually with gitmakedoc
To get started with the SSR and APF have a look at the APF examples and at our coding style guidelines
Discuss problems and new feature proposals in our mailing list
Report issues and propose pull requests on github