Review/extend DEVELOPMENT documentation

[dholbach]

Each of the controllers should have clear guidelines on how to build and run the
projects. This includes the sections:
○ How to install dependencies
○ How to install the controller
○ How to run the test suite locally
○ How to run the controller locally

Some of the information might already be there - let's double-check it's up to date and accurate.

[dholbach]

Please note this file has moved to https://github.com/fluxcd/notification-controller/blob/main/DEVELOPMENT.md now

[dholbach]

This issue might be well-suited for new contributors. Basically the ask is: if you have a fairly clean installation of whatever your OS is: let us know if the instructions in the file above were everything that's needed to get you going on the individual tasks. Please report back if you ran into issues and the docs should be clearer (or make a PR!). Thanks a lot in advance!

[yiannistri]

I'd like to have a go at this.