adding README.md for jsontools

plugins/jsontools/README.md

+# jsontools
+
+# Plugin name
+
+A handy command line tool for dealing with json data.
+
+## Usage
+
+```sh
+# curl json data and pretty print the results
+curl https://coderwall.com/bobwilliams.json | pp_json
+
+# pretty print the contents of an existing json file
+less data.json | pp_json
+
+# json data directly from the command line
+echo '{"b":2, "a":1}' | pp_json
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