Add supplemental information to contribution guideline

[autholykos]

Describe what kind of specification you want to have created
The CONTRIBUTING.md could benefit from:

• A TOC
• A direct link to opening issues ../issues/new
• Some basic Go/Rust/Javascript styleguides
• Some more information about how to communicate with the team (Discord, mailinglist, etc).
• Some high level information for different kind of contributions

For inspiration see Atom's and RoR contribution guidelines.