Clear contribution/development instructions

[lizzyd710]

Maybe I'm just illiterate, but I want to work on an issue (#153) and have no idea how to actually work on this repo. I know I will need iD, and the README references iD's documentation, but I don't know how to test local changes made to this repo, the tagging schema specifically, since it's separate from the actual editor.

Again, I might just be missing something, but I think having clear instructions as to how to contribute to this repository specifically would help in getting more people involved.

[tyrasd]

Sadly, it's quite the opposite of easy at the moment to test the presets. See #289 for some possible improvements and #131 (comment) for a hacky solution.

Closing here in favour of #289