Import Geant4 10.4.3 source tree

CONTRIBUTING.md

-The Geant4 Collaboration:
-http://cern.ch/geant4/collaboration/members
+Contributing to Geant4
+=====
+To contribute patches to Geant4, users can submit Pull Requests (PR) such that:
+
+- A single PR refers to a single particular topic (fix or suggested enhancement);
+  PRs including uncorrelated changes in codes in different areas or modules will
+  not be considered.
+
+A PR will be examined by a responsible and eventually extracted for a patch in
+the development repository of Geant4, to be included in a future release or
+patch.
+
+For simple bug-fixes, it is anyhow recommended to use the official
+`Bugzilla problem reporting system <https://bugzilla-geant4.kek.jp/>`.
+
+The Geant4 Collaboration