Add developer documentation (howto add new export formats)

[jgraup]

Do you have a primer on how to add different export modules? Or basically what the standard input looks like? It would also be nice to include how to build a local version of this project for testing.

Thanks in advance!

[FlorianLudwig]

Hope this helps as a first step:

https://github.com/GreyRook/Draw2Script/blob/master/README.md#manual-installation