document hacking vscode extension and the language server

[Sorin Sbarnea]

Testing modifications locally for to https://github.com/ansible/vscode-ansible and https://github.com/ansible/ansible-language-server should be documented in a development readme, covering both of them as in most cases the language server changes cannot be tested in isolation.

Checklist for things to be covered:

• add a breakpoint to the language server and run extension in vscode debugger ensuring that the breakpoint is used.
• how to use npm link to link the cloned two repositories, ensuring that modifications made to ALS are used by extension.

I already started to write with @webknjaz some of the things we learnt so far https://hackmd.io/GXyun7OZQnqm1YH_6IXlVg but we are far from considering this ready to be added.