[chore]Update CONTRIBUTING.md for public, new signalfx project member, and current signalfx project members

[jvoravong]

Description: More documentation for contributors!

[jinja2]

Suggested change

	As a **public contributor** with no relation to SignalFX organization, you will likely **not have permission to create a feature branch** in the SignalFX repository. Here’s what to do:
	As a **public contributor** with no relation to Signalfx github organization, you will likely **not have permission to create a feature branch** in this repository. Here’s what to do:

We are referring to the github org here right? I think we should use Cisco/Splunk other wise

[jvoravong]

• I mean the signalfx github org, I'm trying to be concise with wording here and elsewhere. Let me take another stab at it.
• I would like to hold off on introducing "Cisco" language until a separate PR where we can update the documentation in the whole repo in a single PR.

[jvoravong]

In addition to the docs changes here, I'm going to add a test (in pre-commit?) to validate people submit PRs properly to this repo. If the test fails it will point them to these new docs.