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

[isc-npetroce]

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 <repo path> 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)