Documented the use of the "subdirectory" parameter (#5949)

* Documented the use of the "subdirectory" parameter

* Moved the subdir documentation up

* Update docs/dependency-specification.md

Co-authored-by: Mathieu Kniewallner <mathieu.kniewallner@gmail.com>

* Added @mkniewallner editorial changes

Co-authored-by: Mathieu Kniewallner <mathieu.kniewallner@gmail.com>

* Fixed final URL

Co-authored-by: Mathieu Kniewallner <mathieu.kniewallner@gmail.com>

Co-authored-by: Mathieu Kniewallner <mathieu.kniewallner@gmail.com>