docs: Indicate support for asyncclick

Signed-off-by: Stephen Finucane <stephen@that.guru>

Author: stephenfin

docs/installation.rst

@@ -14,7 +14,13 @@ Alternatively, install from source by cloning this repo then running
 
    $ python setup.py install
 
+*sphinx-click* supports both `click`__ and `asyncclick`__. If *asyncclick* is
+found, it will be preferred.
+
 .. important::
 
    Both the package you're referencing and any dependencies **must be
    installed**.
+
+.. __: https://pypi.org/project/click/
+.. __: https://pypi.org/project/asyncclick/