Add instructions for `deps.edn` projects

[rads]

The docs make it look like it's a Leiningen-only plugin, but it works fine with the latest Clojure tooling:

$ clojure \
    -Sdeps '{:deps {marginalia/marginalia {:mvn/version "0.9.1"}}}' \
    -M -m marginalia.main

Generating Marginalia documentation for the following source files:
   /Users/rads/biff-lit/src/com/example/test.clj
   /Users/rads/biff-lit/src/com/example/worker.clj
   /Users/rads/biff-lit/src/com/example/schema.clj
   /Users/rads/biff-lit/src/com/example/repl.clj
   /Users/rads/biff-lit/src/com/example/ui.clj
   /Users/rads/biff-lit/src/com/example/middleware.clj
   /Users/rads/biff-lit/src/com/example/email.clj
   /Users/rads/biff-lit/src/com/example/app.clj
   /Users/rads/biff-lit/src/com/example/settings.clj
   /Users/rads/biff-lit/src/com/example/home.clj
   /Users/rads/biff-lit/src/com/example.clj

Done generating your documentation in ./docs