docs: Add an intro note to the maintainers handbook

...just enough to say what the purpose is and to solicit more
contributions.
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/maintainer/index.rst

@@ -2,6 +2,10 @@
 Kernel Maintainer Handbook
 ==========================
 
+This document is the humble beginning of a manual for kernel maintainers.
+There is a lot yet to go here!  Please feel free to propose (and write)
+additions to this manual.
+
 .. toctree::
    :maxdepth: 2