Document how to setup dev environment

[C3pa]

Issue Description

Quoting @carsakiller from LuaLS/LuaLS.github.io#35 (comment):

@sumneko would you be able to provide some more insight into how you develop the language server? I think I recall you saying that development is primarily test-driven. However, I'm not sure how the testing framework works — I believe it is custom? Some pointers on writing and running tests would be appreciated!

Is there a specific flow/method you would recommend for contributors? For the few simple things I have contributed, I have just used print() as described in this PR — but I don't think it would be very efficient for larger changes.

I think creating good documentation here for developing the language server would greatly help the community make some really nice pull requests. I've seen many comments from users eager to help but confused when getting their development environment set up and navigating the codebase.

Some initial work was recently merged in LuaLS/LuaLS.github.io#35.

Additional Notes

No response