Add a developer "getting started" page

[berry120]

At present there's no "developer" documentation for cloning Quelea from Github, opening it in an IDE and running it for the first time. It would be good to have a page added that covers this type of "getting started" documentation from a developer perspective.

IntelliJ should be used as the example IDE where relevant, as that's by far the most popular Java IDE these days, and also the one the main developers use.

[rushikeshadhav]

I would like to contribute. Can you tell me what steps do I have to include in the documentation.

[joncojonathan]

Note there are some starting steps at https://github.com/quelea-projection/Quelea/blob/master/docs/SETUP.md which may be a useful starting point.

I've submitted a PR to update those (quelea-projection/Quelea#508) as the instructions were out of date.

Longer term it's probably better to put a pointer in the main repo's SETUP.md to direct people to a developer section on the docs site.