add ./configure --help and a pointer to the reference manual

README

@@ -8,25 +8,30 @@ To install Gmsh, type
 make
 make install
 
-This requires the GSL (version 1.2 or higher; freely available from
-http://sources.redhat.com/gsl/) and FLTK (version 1.1.x, configured
-with OpenGL support; freely available from http://www.fltk.org) to be
-installed on your system. If not in default locations, you can either
-use the --with-fltk-prefix and --with-gsl-prefix configure options, or
-you can set the environment variables FLTK_DIR and GSL_DIR.  Compiling
-the Windows version requires the Cygwin tools (freely available from
-http://www.cygwin.com).
-
-To install a non-graphical version of Gmsh (that does not require
-FLTK nor OpenGL), type
+This requires GSL 1.2 or higher (freely available from
+http://sources.redhat.com/gsl/) and FLTK version 1.1.x (configured
+with OpenGL support; freely available from http://www.fltk.org). You
+can use the --with-fltk-prefix and --with-gsl-prefix configure options
+(or define the FLTK_DIR and GSL_DIR environment variables) if the
+libraries are not installed in their default locations. Please note
+that compiling the Windows version requires the Cygwin tools (freely
+available from http://www.cygwin.com).
+
+To install a non-graphical version of Gmsh (that does not require FLTK
+nor OpenGL), type
 
 ./configure --disable-gui
 make
 make install
 
+For a description of all other configuration options, type
+
+./configure --help
+
 Gmsh is distributed under the terms of the GNU General Public
 License. See doc/COPYING for more information.
 
-See the doc/ and tutorial/ directories for documentation.
+See the doc/ and tutorial/ directories for documentation. The
+reference manual is located in doc/texinfo/.
 
 See the demos/ directory for additional examples.