Added example with DUMPMASSCHARGE

[makedoc]

src/generic/DumpMassCharge.cpp

@@ -40,6 +40,10 @@ This command dumps a file containing charges and masses.
 It does so only once in the simulation (at first step).
 File can be recycled in the \ref driver tool.
 
+Notice that masses and charges are only written once at the beginning
+of the simulation. In case no atom list is provided, charges and
+masses for all atoms are written.
+
 \par Examples
 
 You can add the DUMPMASSCHARGE action at the end of the plumed.dat
@@ -61,6 +65,31 @@ add the --mc flag on the driver command line, e.g.
 plumed driver --mc mcfile --plumed plumed.dat --ixyz traj.xyz
 \endverbatim
 
+With the following input you can dump only the charges for a specific
+group.
+\verbatim
+solute_ions: GROUP ATOMS=1-121,200-2012
+DUMPATOMS FILE=traj.gro ATOMS=solute_ions STRIDE=100
+DUMPMASSCHARGE FILE=mcfile ATOMS=solute_ions
+\endverbatim
+Notice however that if you want to process the charges
+with the driver (e.g. reading traj.gro) you have to fix atom
+numbers first, e.g. with the script
+\verbatim
+awk 'BEGIN{c=0}{
+  if(match($0,"#")) print ; else {print c,$2,$3; c++}
+}' < mc > newmc
+}'
+then
+\verbatim
+plumed driver --mc newmc --plumed plumed.dat --ixyz traj.gro
+\endverbatim
+
+\endverbatim
+
+
+
+
 */
 //+ENDPLUMEDOC