Update contributing.md to describe how to build and test locally

[rjrudin]

The current CONTRIBUTING.md file only describes how to submit a pull request. What we need are instructions for the following:

1. How to build and test the plugin locally in VSCode
2. How to run the tests (which I assume should include what's at ./client/test and at ./server/test

The above will allow any developer to review/approve future PR's by manually testing the plugin locally. And we can soon get a Jenkins build together for running the automated tests on every PR and on the develop branch when it's updated.

For bonus points - include some info about what the "client" and "server" folders are for, in particular the latter - I wasn't expecting to see a "server" folder in a VSCode plugin.

[bshalke]

My understanding of server is that it's the language server used to do syntax highlighting. I'll take a crack at this.