Adds json usage example.

lib/pure/json.nim

@@ -13,6 +13,23 @@
 ## (unlike XML). It is easy for machines to parse and generate.
 ## JSON is based on a subset of the JavaScript Programming Language,
 ## Standard ECMA-262 3rd Edition - December 1999.
+##
+## Usage example:
+##
+## .. code-block:: nimrod
+##  let
+##    small_json = """{"test": 1.3, "key2": true}"""
+##    jobj = parseJson(small_json)
+##  assert (jobj.kind == JObject)
+##  echo($jobj["test"].fnum)
+##  echo($jobj["key2"].bval)
+##
+## Results in:
+##
+## .. code-block:: nimrod
+##
+##   1.3000000000000000e+00
+##   true
 
 import 
   hashes, strutils, lexbase, streams, unicode