better docs for make binaries

Signed-off-by: Riyaz Faizullabhoy <riyaz.faizullabhoy@docker.com>

README.md

@@ -88,3 +88,13 @@ Prerequisites:
     - Mac OS ([Homebrew](http://brew.sh/)): `brew install libtool`
 
 Run `make binaries`, which creates the Notary Client CLI binary at `bin/notary`.
+Note that `make binaries` assumes a standard Go directory structure, in which
+Notary is checked out to the `src` directory in your `GOPATH`. For example:
+```
+GOPATH=$(<path to go/>)
+go/
+    src/
+        github.com/
+            docker/
+                notary/
+```