Improve installation documentation

Co-authored-by: Adina Wagner <adina.wagner@t-online.de>

docs/source/installation.rst

@@ -34,8 +34,10 @@ and a gooey-only installer. The full
 installer will install ``datalad-gooey`` as well as  ``git`` and ``git-annex``.
 It requires admin privileges to execute successfully. The gooey-only installer
 will only install ``datalad-gooey``. It can be executed as admin user or as
-non-admin user. If you use the gooey-only installer, ``git`` and ``git-annex`` have
-to be provided by other means, e.g. an administrator installs them.
+non-admin user. If you use the gooey-only installer, ensure that ``git`` and ``git-annex`` have
+been installed on your system already. For more information, check their respective
+installation instructions at `gitforwindows.org <https://gitforwindows.org>`_ and 
+`git-annex.branchable.com/install/Windows <https://git-annex.branchable.com/install/Windows/>`_.
 
 The installers can be downloaded `here`_.