Add new documentation

[john-light]

(Normally I wouldn't put so much in one PR but this was the easiest way to fix an earlier git mistake.)

• Contributing guidelines
• How to verify cryptographic hashes and signatures
• How to vote in an Aragon Network Vote

[Smokyish]

@john-light I pushed a small change, please have a look and move the PR from Draft to Ready for review when done.

[john-light]

@Smokyish this is ready to merge