Added dependency for server edition and reference to ppa in build instructions.

README

@@ -51,6 +51,9 @@ sudo apt-get install -y git-core
 # basic libraries necessary for both DUNE and OPM
 sudo apt-get install -y libboost-all-dev libsuperlu3-dev libsuitesparse-dev
 
+# for server edition of Ubuntu add-apt-repository depends on
+sudo apt-get install python-software-properties
+
 # add this repository for necessary backports (required for Ubuntu Precise)
 sudo add-apt-repository -y ppa:opm/ppa
 sudo apt-get update
@@ -103,7 +106,8 @@ or (if installing to /usr/local or similar)
 2. As a dune module.
  - Put the opm-core directory in the same directory
    as the other dune modules to be built (e.g. dune-commmon,
-   dune-grid).
+   dune-grid). Note that for Ubuntu you can install Dune
+   from the ppa as outlined above.
  - Run dunecontrol as normal. For more information on
    the dune build system, see
    http://www.dune-project.org/doc/installation-notes.html