Add sample for exec plugin. Fixes influxdata#245

plugins/exec/README.md

@@ -1,7 +1,6 @@
 # Exec Plugin
 
-The exec plugin can execute arbitrary commands which return flattened
-JSON.
+The exec plugin can execute arbitrary commands which output JSON. Then it flattens JSON and finds all float values.
 
 For example, if you have a json-returning command called mycollector, you could
 setup the exec plugin with:
@@ -18,3 +17,23 @@ The name is used as a prefix for the measurements.
 The interval is used to determine how often a particular command should be run. Each
 time the exec plugin runs, it will only run a particular command if it has been at least
 `interval` seconds since the exec plugin last ran the command.
+
+
+# Sample
+
+Let's say that we have a command which gives the following output:
+```json
+{
+    "a": 0.5,
+    "b": {
+        "c": "some text",
+        "d": 0.1
+    }
+}
+```
+
+The collected metrics will be:
+```
+exec_catter_a value=0.5
+exec_catter_b_d value=0.1
+```