Document additional usage of json resource

The `command` and `content` usage seems to have been supported for some time, but is not included in the documentation.

ref:
* https://github.com/inspec/inspec/pull/1271

Signed-off-by: Sean Escriva <sean.escriva@gmail.com>

docs/resources/json.md.erb

@@ -47,6 +47,21 @@ where
 * `name` is a configuration setting in a JSON file
 * `should eq 'foo'` tests a value of `name` as read from a JSON file versus the value declared in the test
 
+The `json` resource can also be used with json formatted output from a command. 
+Using the same json as the previous exmaple, it can be queried using:
+
+    describe json({ command: 'retrieve_data.py --json'}) do
+      its('name') { should eq 'hello' }
+      its(['meta','creator']) { should eq 'John Doe' }
+      its(['array', 1]) { should eq 'one' }
+    end
+
+Finally content can be passed directly to the resource:
+
+    describe json({ content: '{\"item1\": { \"status\": \"available\" } }' }) do
+      its(['item1', 'status']) { should cmp 'available' }
+    end
+
 <br>
 
 ## Examples