core/core: Refer back to the main go-ipfs docs

So folks looking in core can figure out how it fits into the
repository as a whole.

core/core.go

@@ -2,7 +2,10 @@
 Package core implements the IpfsNode object and related methods.
 
 Packages underneath core/ provide a (relatively) stable, low-level API
-to carry out most IPFS-related tasks.
+to carry out most IPFS-related tasks.  For more details on the other
+interfaces and how core/... fits into the bigger IPFS picture, see:
+
+  $ godoc github.com/ipfs/go-ipfs
 */
 package core