Add pkg doc

codec/std/doc.go

@@ -0,0 +1,10 @@
+/*
+Package std defines all the common and standard Cosmos SDK Protocol Buffer
+message definitions and type implementations. The std package defines and exposes
+a single concrete Codec type that implements all the necessary module codec
+interfaces along with all the types necessary to use those modules.
+
+Applications can choose to either use the codec directly or even extend it without
+having the need to redefine all the message and type implementations.
+*/
+package std