Add `dir' file to list the info manual

If a user doesn't want to install geiser, it's possible to run it in
place by loading `elisp/geiser.el' directly.

It's also possible to read the info manual directly but it's
inconvenient to not have it listed when hitting C-h i inside Emacs.

To list such manual, it's necessary to tell info where to look for it.
One way of doing it is adding the directory to the variable
`Info-additional-directory-list'.  But, for this to work, there's need
to be a `dir' file listing the info manual.