devmapper: document the default DM thin pool blocksize

Docker-DCO-1.1-Signed-off-by: Mike Snitzer <snitzer@redhat.com> (github: snitm)

daemon/graphdriver/devmapper/README.md

@@ -128,7 +128,8 @@ Here is the list of supported options:
 
  *  `dm.blocksize`
 
-    Specifies a custom blocksize to use for the thin pool.
+    Specifies a custom blocksize to use for the thin pool.  The default
+    blocksize is 512K.
 
     Example use: