update README

README.md

@@ -19,7 +19,7 @@ gnuplot (www.gnuplot.info/)
 ${SRC_DIR}/recursive_gmconvert.sh -f map_filename -t threshold -n num_gaussians -N num_iterations [-i first_iteration] [-s]
 ```
 
-Required arguments:
+###Required arguments:
 
 `-f map_filename`: the file name of the input EM map
 
@@ -29,7 +29,7 @@ Required arguments:
 
 `-N num_iterations`: number of recursion levels
 
-Optional arguments:
+###Optional arguments:
 
 `-i first_iteration`: initial recursion level (defaults to 1)