Update readme to mention landmasks

Author: joferkington

utils/README

@@ -1,4 +1,10 @@
-The border/coastline data used by basemap can be updated it two ways:
+Utilities for Updating Coastline/Border Data
+============================================
+
+This folder contains utilities for updating basemap's version of GMT's GSHHS
+shoreline, river, and political border dataset.
+
+The coastline, etc data used by basemap can be updated in two ways:
 
 A) Download the GSHHS shapefiles and use them
 B) Use GMT if you already have it installed, and avoid the download.
@@ -25,3 +31,14 @@ Option B - Generic Mapping Tools
 3) run dumpbounds.sh to create states, countries, rivers text files.
 4) run readboundaries.py to process into binary files, with metadata in
    separate text files.
+
+Updating Landmask Data
+----------------------
+
+In most cases, you will not need to update the raster landmasks used by
+basemap.  However, if GSHHS has had a large number of new islands or lakes
+added, you may want to update the landmasks in addition to the vector data.
+
+You'll need GMT installed to update the landmasks. Once you have it (and the
+full-resolution GSHHS data, which is usually a separate package), run
+``update_landmasks.py``.