add README

README.md

+# Poisson problem
+
+The Poisson problem is a simple partial differentual equation (PDE) that is at the
+core of many applications. The example is useful for desccribing the features
+of MPI/OpenMP that can be used in solving partial differential equations and
+other problems that involve decomposition across many processes.
+
+Authors: Andrea Mignone   (mignone@to.infn.it)       ---> original version
+         Valentina Cesare (valentina.cesare@inaf.it) ---> developer
+         David Goz        (david.goz@inaf.it)        ---> developer
+	 Giulio Malenza   (giulio.malenza@unito.it)  ---> developer
+
+Date   : June 2024
\ No newline at end of file