ENH: README.txt: notes

README.txt

+2013-01
+
+- to UPstream added allocation of communicators
+- added communicators to lduMesh,lduInterface and (indirectly)
+  to lduInterfaceField
+- gamg agglomeration allocates new communicator for every level
+- in all linear solvers/smoothers/preconditioners make they use
+  communicator
+- added lots of warnings if using unexpected communicator (UPstream::warnComm)
+- did LUScalarMatrix for 'directSolveCoarsest'