Add doc on how to sign things electronically?

[iteles]

As per my comment in #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.

[ghost]

@iteles what's the priority for this one?

[iteles]

@markwilliamfirth It's just a question with a yes, no answer. Didn't think to add a priority before we'd decided to do it.

If you've already read it, do you reckon it's worth adding here?

[ghost]

Sorry i thought you also wanted it implementing too

I've never had to use it, but if you think it's useful and you want it here then I don't see why you wouldn't do that

[iteles]

Grand, let's get it in then.