Add README.md

Just a initial one. Needs to be improved.

README.md

+# (UNDER DEVELOPMENT) Neutron Detector VMM module
+
+The VMM module orchestrates the operation of so-called Hybrids, which are a collection of two VMM ASICS together.
+
+# Compilation
+
+This VMM version need the [RMM module](https://gitlab.esss.lu.se/nice/dev-epics-modules/rmm.git) installed in your environment particularly in the branch udp_core. The branch udp_core is not merged into main yet because it works only with most recent versions of the firmware, and some older versions are still in production.