Add Discussions in Contributing Guide

[ArielSAdamsNASA]

Checklist (Please check before submitting)

• I reviewed the Contributing Guide.
• I reviewed the cFS README.md file to see if the feature is in the major future work.
• I performed a cursory search to see if the feature request is relevant, not redundant, nor in conflict with other tickets.

Is your feature request related to a problem? Please describe.
cFS now uses Discussions as a way to ask questions or request information. This is not in the contributing guide.

Describe the solution you'd like
Add that users may start discussions or ask questions in the Discussions tab of cFS under Ways to Contribute in the Contributing Guide.

Requester Info
Ariel Adams, ASRC Federal