Write developer docs on debugging language server issues

[charlieegan3]

#632 and open-policy-agent/vscode-opa#147 make some changes that encode some hard earned learnings about how the LSP works. It'd be good to write up some notes on the messages and how they are handled on the server and the client. In particular:

• the notion of requests vs notifications
• that the server and the client can both send requests
• and that processing a request is blocking on the client and so you need to respond before sending another request.