doc: Add some docs explaining the sub-package layout

Based on [1].

[1]: https://github.com/ipfs/go-ipfs/issues/1158

doc.go

-// IPFS is a global, versioned, peer-to-peer filesystem
+/*
+IPFS is a global, versioned, peer-to-peer filesystem
+
+There are sub-packages within the ipfs package for various low-level
+utilities, which are in turn assembled into:
+
+core/...:
+  The low-level API that gives consumers all the knobs they need,
+  which we try hard to keep stable.
+shell/...:
+  The high-level API that gives consumers easy access to common
+  operations (e.g. create a file node from a reader without wrapping
+  with metadata). We work really hard to keep this stable.
+
+Then on top of the core/... and shell/... Go APIs, we have:
+
+cmd/...:
+  Command-line executables
+test/...:
+  Integration tests, etc.
+
+To avoid cyclic imports, imports should never pull in higher-level
+APIs into a lower-level package.  For example, you could import all of
+core and shell from cmd/... or test/..., but you couldn't import any
+of shell from core/....
+*/
 package ipfs