Add doc on how to sign things electronically?

As per my comment in https://github.com/dwyl/process-handbook/pull/62, I have been asked a number of times how to sign things electronically and have to send people to this document on a somewhat random repo as it's all we have at the moment! https://github.com/dwyl/summer/blob/master/freelance-agreements/readme.md

Shall we add it (or a slightly updated version) to this repo for completion? Should be quick as at least the Mac part is still all correct, just needs a smidge of formatting.

Assigning you @markwilliamfirth as you mentioned electronic signatures in your PR.