papr: add documentation to YAML and bump context

.papr.yml

@@ -1,5 +1,16 @@
 ---
 
+# This YAML file is used by PAPR. It details the test
+# environment to provision and the test procedure. For more
+# information on PAPR, see:
+#
+#   https://github.com/projectatomic/papr
+#
+# The PAPR YAML specification detailing allowed fields can
+# be found at:
+#
+#   https://github.com/projectatomic/papr/blob/master/sample.papr.yml
+
 cluster:
   hosts:
     - name: ocp-master
@@ -19,9 +30,10 @@ packages:
   - openssl-devel
   - redhat-rpm-config
 
-context: 'fedora/25/atomic | origin/v3.6.0-alpha.1'
+context: 'fedora/25/atomic | origin/v3.6.0-alpha.2'
 
 env:
+  # NB: when bumping image version, also bump context above ^
   OPENSHIFT_IMAGE_TAG: v3.6.0-alpha.2
 
 tests: