Add a readme for application developers

[drewcrawford]

Orca has extensive documentation written specifically at application developers, see here.

This is my attempt to start the same kind of document for the same kind of audience. The main questions to address in the document are:

1. What tools are available and which ones should I actually use?
2. How do I start and stop them, and which of the many commands will help me quickly check my application?
3. What are typical problems I will encounter and how do I fix them?

I have filled in this information based on the situations that I was confused about so far.

On the last point it would be good to have an equivalent to the orca 'A Request' section (https://gitlab.gnome.org/GNOME/orca/-/blob/main/README-APPLICATION-DEVELOPERS.md?ref_type=heads&plain=0#a-request) to give application developers a clear call to action of some kind. I am not sure what kind; I have been (ab)using the github discussion feature but it would help for the project as a whole to take a position on what application developers ought to do.