README: note where to find CPU/memory details (#116)

My first question was "how/why is this faster" and the answer is
a little bit buried, so try to make it clearer.

README.md

@@ -35,7 +35,8 @@ updates in change requests.
 
 ## encoding/json [![GoDoc](https://godoc.org/github.com/segmentio/encoding/json?status.svg)](https://godoc.org/github.com/segmentio/encoding/json)
 
-More details about the implementation of this package can be found [here](json/README.md).
+More details about _how_ this package achieves a lower CPU and memory footprint
+can be found [in the package README](json/README.md).
 
 The `json` sub-package provides a re-implementation of the functionalities
 offered by the standard library's [`encoding/json`](https://golang.org/pkg/encoding/json/)