Document steps to set up a local testing environment in CONTRIBUTING.md

Currently we do not offer guidance on how to set up a local testing environment for Embedded Git; we should include this in CONTRIBUTING.md . A sketch of the steps would be

- Install some (minimum supported?) version of IRIS locally, CE is fine
- `git clone` the repository locally
- use `zpm load ` on the local directory to load GSC from local
- Setup VSCode to have local repo open + a server connection to IRIS instance
- VSCode should push changes to IRIS automatically
- might need to install nodejs (verify this)