[docs] Creating contribution guidelines & inline doc templates

As people start to contribute to the repository, it'd be good to have contribution guidelines setup for consistency. The documentation throughout the repository changes subtly as well; having a set template and key information about how to document would be helpful as `essential-eth` migrates away form `README` documentation into exclusively `TypeDoc` documentation compiled through Docusaurus.

I'm feeling a calling to take this one on @dawsbot but you've obviously been maintaining the repo longest; let me know if you'd rather be assigned to this issue.