message: Update PWG-MM README
diff: /*! \page pwgmm PWG-MM
  -... PWG-MM directory under construction ...
  +This directory contains the code relevant for the analyses hosted in
  +PWG-MM. The code is organised according to the PAG structure.
  -# Monte Carlo generators
  +Analyses are discussed on the page \subpage pwgmm_ana.
  -Event generators are discussed on the page \subpage pwgmm_mc.
  +# Luminosity
  -# Analyses
  +# Multiplicity
  -Analyses are discussed on the page \subpage pwgmm_ana.
  +# Rivet & Monte Carlo generators
  +
  +Event generators are discussed on the page \subpage pwgmm_mc.
  +
  +# Underlying Event
  */
  