Added note on supported data requirements for Device Defender (#429)

* Added note on supported data requirements for Device Defender
* Updated note wording

Author: TwistedTwigleg

samples/README.md

@@ -843,3 +843,9 @@ must provide privileges for this sample to connect, publish, and receive.
 }
 </pre>
 </details>
+
+**Note:** The formatting and data requirements for custom metrics in Device Defender are listed below. If you input data that does not follow the requirements, it will be sent in the JSON packet but Device Defender will not show/retain the data! If your data is not showing up in Device Defender, please double check that your data fits into the requirements below.
+
+* `number` and `number-list` types: Supports precision up to 15 digits. Supports both positive and negative values.
+* `string-list` type: Supports letters A through Z (uppercase and lowercase) and the characters `:`, `_`, `-`, `/`, and `.
+* `ip-list` type: Supports only valid IPV4 and IPV6 addresses.