Update CONTRIBUTING.md to Organization Wide Use

[kb1lqc]

I copied our old faraday-software document to this repository. It should be updated to fix the following at a minimum.

• Remove old links such as to the Slack channel, replace with Gitter.
• Change recommended programs
• Change references to faraday-software specific use
• Add desired GitFlow use
• Model better after the Atom project (I know, vague)

@kb1lqd @reillyeon @jbemenheiser @hdkmike @el-iso for awareness