Added ref to openmm-plumed

[makedoc]

user-doc/Intro.txt

@@ -68,6 +68,7 @@ As far as we know, the following MD codes can be used with PLUMED 2 out of the b
 - [PINY-MD](http://github.com/TuckermanGroup/PINY), in its plumed branch.
 - [IPHIGENIE](http://sourceforge.net/projects/iphigenie/).
 - [AceMD](http://www.multiscalelab.org/acemd/).
+- [OpenMM](http://openmm.org), using the [openmmp-plumed plugin](http://github.com/peastman/openmm-plumed).
 
 Please refer to the documentation of the MD code to know how to use it with the latest PLUMED release.
 If you maintain another MD code that is PLUMED-ready let us know and we will add it to this list.