Feature/add dtap docs

Add documentation to the DTAP section

docs/architecture/dtap/acceptance.md

 # Acceptance environment
 
-TBA
+Once GAP components have been tested successfully in the test environment, they
+are advanced to the UAT environment. In this environment, stakeholders from
+other teams such as Network Engineering and Operations are able to perform their
+own tests on the system.
+
+In the UAT environment, only full releases are deployed. This means that
+development builds are not included in this environment. Devices targeted by GAP
+are physical routers in the lab, to increase accuracy of integration testing.
+
+The UAT environment has been designed to mimic the ultimate production
+environment as close as possible. To achieve this, all VMs are managed in the
+same manner using Puppet, and external resources are configured identically. The
+external service database is hosted on a distributed Postgres cluster, which is
+provided by the DevOps team. The Redis instance is also a cluster, which again
+comes from the DevOps team.
+
+Once testing has been completed, components are ready to move to the production
+environment.