Update CONTRIBUTING to have a more clear intro to project scripts

[Ellis Wright]

While working on a PR for the project @izgeri mentioned that she was surprised at how well fleshed out the projects scripts for testing/generating a client were and that the CONTRIBUTING document was lacking a good enough intro to those scripts for a new contributor.

AC:

• Add a "Getting Started" section at the beginning of the document which touches on the scripts and links to the relevant utility scripts section
• Expand descriptions for scripts which are currently lacking in detail