Add information on information exclusivity.

doc/security/README.md

@@ -2,3 +2,4 @@
 - [Password length limits](password_length_limits.md)
 - [Rack attack](rack_attack.md)
+- [Information exclusivity](information_exclusivity.md)

doc/security/information_exclusivity.md

+# Information exclusivity
+
+Git is a distributed version control system (DVCS).
+This means that everyone that works with the source code has a local copy of the complete repository.
+In GitLab every project member that is not a guest (so reporters, developers and masters) can clone the repository to get a local copy.
+After obtaining this local copy the user can upload the full repository anywhere, including another project under their control or another server.
+The consequense is that you can't build access controls that prevent the intentional sharing of source code by users that have access to the source code.
+This is an inherent feature of a DVCS and all git management systems have this limitation.