Add comment about Prometheus vendoring constraint

Author: juliusv

Gopkg.toml

@@ -10,6 +10,11 @@
   name = "github.com/weaveworks/promrus"
   version = "v1.1.0-legacy"
 
+# Since Cortex is heavily based on Prometheus packages, we want
+# to track upstream master quite closely, without being constrained
+# to the latest Prometheus release. This means we get various bug
+# fixes and features earlier, and makes necessary refactorings upon
+# vendor updates less huge (if updated frequently).
 [[constraint]]
   name = "github.com/prometheus/prometheus"
   branch = "master"