drm/doc: Add TODO list

This commit adds a TODO list to the GPU driver developer's guide. The
content was taken from the DRMJanitors page on the X.Org wiki:

	https://www.x.org/wiki/DRMJanitors/

The goal is to track a list of refactorings that would be nice to see
merged eventually. Sometimes these would be encountered during patch
review on the mailing list, and at other times one can come across
these while working in a specific area of code.
Signed-off-by: Thierry Reding <treding@nvidia.com>
Acked-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20170207175113.2793-1-thierry.reding@gmail.com

Documentation/gpu/index.rst

@@ -13,6 +13,7 @@ Linux GPU Driver Developer's Guide
    i915
    vga-switcheroo
    vgaarbiter
+   todo
 
 .. only::  subproject and html
 

Documentation/gpu/introduction.rst

@@ -50,3 +50,13 @@ names are "Notes" with information for dangerous or tricky corner cases,
 and "FIXME" where the interface could be cleaned up.
 
 Also read the :ref:`guidelines for the kernel documentation at large <doc_guide>`.
+
+Getting Started
+===============
+
+Developers interested in helping out with the DRM subsystem are very welcome.
+Often people will resort to sending in patches for various issues reported by
+checkpatch or sparse. We welcome such contributions.
+
+Anyone looking to kick it up a notch can find a list of janitorial tasks on
+the :ref:`TODO list <todo>`.