Included logmfd module documentation

provided by @tetsuyamorishita

Closes #422

user-doc/LOGMFDMOD.md

-\page LOGMFDMOD xxxxxxxx
+\page LOGMFDMOD Logarithmic Mean Force Dynamics
 
 <!-- 
-description: xxxxxxxxx
-authors: xxxxxxxxx
-reference: xxxxxxxxxx
+description: Method for enhanced sampling and for free energy calculations along collective variables
+authors: Tetsuya Morishita, Naoki Watanabe
+reference: \cite MorishitaLogMFD \cite MorishitaLogPD \cite MorishitaVsLogMFD
 -->
 
 ## Overview
 
-xxxxxxxxx
+The LOGMFD module contains the LogMFD/LogPD method for enhanced sampling in a CV space and for on-the-fly free energy reconstruction along the CVs. This module implements the multiple-replica algorithm (LogPD \cite MorishitaLogPD) as well as the single-replica algorithm (LogMFD \cite MorishitaLogMFD), the former invoking the Crooks-Jarzynski nonequilibrium work relation. In addition, TAMD/d-AFED \cite AbramsJ2008 can also be implemented by this module.
 
 ## Installation 
 This module is not installed by default. Add '\-\-enable-modules=logmfd' to your './configure' command when building PLUMED to enable these features.