Don't recommend pulling straight from `main` for adding/updating the subtree

[robojumper]

README.md has commands that pull from latest main or recommends downloading the latest sources to copy them into the project. We should instead have a stable branch for this purpose wo that the regular installation process can only end up installing a release version, not a development version that may be in flux.

May become obsolete with #25 but that's still a ways off.

[Xymanek]

I would suggest instead prompting users to use a specific tag (instead of a branch name)