Add CONTRIBUTING.md

[tqe0]

The repo has no contributor guide, so new contributors have to work out the
setup, test and lint commands from the README and CI config, and often miss
two things entirely: the Palantir CLA, and that PRs go to develop rather
than master.

This adds a CONTRIBUTING.md covering:

• signing the CLA at cla.palantir.com
• local setup and running pytest / pylint
• basing work on develop
• where plugins live and how to register them
• manual testing against the VS Code client
• what to include in a bug report

Docs only — no code changes made.

[palantirtech]

Thanks for your interest in palantir/python-language-server, @tqe0! Before we can accept your pull request, you need to sign our contributor license agreement - just visit https://cla.palantir.com/ and follow the instructions. Once you sign, I'll automatically update this pull request.