Added HW requirements to Readme.md

readme.md

@@ -23,7 +23,15 @@ Each language SDK includes sample code and documentation in addition to the libr
 
 ### OS platforms and hardware compatibility
 
-Azure IoT device SDKs can be used with a broad range of OS platforms and devices. See [OS Platforms and hardware compatibility](https://azure.microsoft.com/documentation/articles/iot-hub-tested-configurations/).
+Azure IoT device SDKs can be used with a broad range of OS platforms and devices. The minimum requirements are for the device platform to support the following:
+
+- Being capable of establishing an IP connection: only IP-capable devices can communicate directly with Azure IoT Hub.
+- Support TLS: required to establish a secure communication channel with Azure IoT Hub.
+- Support SHA-256: necessary to generate the secure token for authenticating the device with the service.
+- Have a Real Time Clock or implement code to connect to an NTP server: necessary for both establishing the TLS connection and generating the secure token for authentication.
+- Having at least 64KB of RAM: the memory footprint of the SDK depends on the SDK and protocol used as well as the platform targeted. The smallest footprint is achieved using the C SDK targeting microcontrollers.
+
+You can find an exhaustive list of the OS platforms the various SDKs have been tested against on our [OS Platforms and hardware compatibility](https://azure.microsoft.com/documentation/articles/iot-hub-tested-configurations/) page. Note that you might still be able to use the SDKs on OS and hardware platforms that are not listed on this page: all the SDKs are open sourced and designed to be portable. If you have suggestions, feedback or issues to report, refer to the Contribution and Support sections below.
 
 ## Microsoft Azure IoT service SDKs