Document project-roots.

[wigust]

This commit fixes tests on Emacs 27.1 (fixes #568).

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[wigust]

Also, please consider creating a new release by pushing a Git tag, so it could be used in Guix instead of pointing to commit.

[manuel-uberti]

IIUC, in project.el 0.5.2 has been made obsolete in favour of project-root. Would it make sense to use and document that one instead?

[bbatsov]

IIUC, in project.el 0.5.2 has been made obsolete in favour of project-root. Would it make sense to use and document that one instead?

clojure-mode doesn't depend on project.el as a package, so it will use whatever version comes bundled with Emacs, so I don't think we can switch to the new name just yet. Better compatibility trumps an obsoletion warning in my book.