Add contributing to DrupalPod documentation

[mradcliffe]

Is your feature request related to a problem? Please describe

I find it hard to remember some of the required query parameters on an existing gitpod instance created from a pull request. It would also be nice to have a getting started making a pull request for DrupalPod.

Describe the solution you'd like

Adds a CONTRIBUTING.md file to the repository that documents some helpful commands and gitpod.io query parameters used by DrupalPod like DP_PROJECT_TYPE, GITPOD_HEADLESS, DP_CORE_VERSION, DP_MODULE_VERSION and when to use them.

Describe alternatives you've considered

Additional context