core/corehttp/corehttp: Add a package comment

I'm not entirely clear on the role that this package is filling, but
this description seems like a reasonable guess based on a quick skim
through it's exported API.

core/corehttp/corehttp.go

+/*
+Package corehttp provides utilities for the webui, gateways, and other
+high-level HTTP interfaces to IPFS.
+*/
 package corehttp
 
 import (