Document usage of `install-from-souce` dev script

[jotaen4tinypilot]

The usage of the install-from-source dev script is not really documented anywhere, and we could also make it easier to discover.

We should add a section to our contribution guide lines that describes the usage of the script, including:

• The suggested setup around SSH keys for cloning the Pro repo
• A code sample that demonstrates the entire process, e.g. to which location the repo should be cloned, or whether there are any other steps that have to be carried out beforehand