added faq entry about erroneous chunks causing blank maps

This seems to come up often enough to warrent an FAQ entry. Wrote up an
explanation of what happen and what to do.

docs/faq.rst

@@ -164,3 +164,31 @@ two ways.
 3. The third non-option is to not worry about it. The problem will fix itself if
    people explore near there, because that will force that part of the map to
    update.
+
+My map is zoomed out so far that it looks (almost) blank
+--------------------------------------------------------
+
+We see this quite a bit, and seems to stem from a bug in the Minecraft terrain
+generation.
+
+Explanation: Minecraft generates chunks of your world as it needs them. When
+Overviewer goes to render your map, it looks at how big the world is, and
+calculates how big the maps needs to be in order to fit it all in.
+Occasionally, we see that Minecraft has generated a few chunks of the world
+extremely far away from the main part of the world. These erroneous chunks have
+most likely not been explored [*]_ and should not exist.
+
+There are two solutions. The preferred is to delete the offending chunks. Open
+up your region folder of your world and look at the region file names. They are
+numbered ``r.##.##.mcr`` where ``##`` is a number. The two numbers indicate the
+coordinates of that region file. Look for region files with coordinates much
+larger in magnitude than any others. Most likely you will find around 1–3
+region files with coordinates much larger than any others. Delete or otherwise
+remove those files, and re-render your map.
+
+The other option is to use the :ref:`crop<crop>` option to tell Overviewer not
+to render all of your map, but instead to only render the specified region.
+
+As always, if you need assistance, come chat with us on :ref:`irc<help>`.
+
+.. [*] They could also have been triggered by an accidential teleport where the coordinates were typed in manually.