Add explanation on working with secure ssh-keys

[surchs]

There are places with good docs on how to use ssh with keys, e.g. GH. But we should be explicit about our own requirements and link to these.

Some points:

• create ssh key
• protect with passphrase
• use keychain / password manager or lose key
• other?

Transferred from https://github.com/DouglasNeuroInformatics/Douglas_Internal_Processes/issues/48

[surchs]

@all-contributors please add @surchs for ideas, content, documentation, projectManagement

[allcontributors]

@surchs

I've put up a pull request to add @surchs! 🎉