Add intro to Platform.md

Update with intro from PR #463.

Author: Amanda Butler

docs/reference/configuration/Platform.md

@@ -1,5 +1,7 @@
 <h2 id="configuration-platform">Platform</h2>
 
+The following is the complete list of platform configuration parameters, as generated by `mbed compile --config -v`. Please see [the configuration system documentation](/docs/development/reference/configuration.html) for details on how you may use or override these settings.
+
 ```
 Configuration parameters
 ------------------------