Update readme with Silicon Labs kit support

Author: stevew817

README.md

@@ -85,6 +85,13 @@ Nordic:
 Renesas:
 * [RZ-A1H](http://developer.mbed.org/platforms/Renesas-GR-PEACH/) (Cortex-A9)
 
+Silicon Labs:
+* [EFM32 Zero Gecko] (https://developer.mbed.org/platforms/EFM32-Zero-Gecko/) (Cortex-M0+)
+* [EFM32 Happy Gecko] (https://developer.mbed.org/platforms/EFM32-Happy-Gecko/) (Cortex-M0+)
+* [EFM32 Leopard Gecko] (https://developer.mbed.org/platforms/EFM32-Leopard-Gecko/) (Cortex-M3)
+* [EFM32 Giant Gecko] (https://developer.mbed.org/platforms/EFM32-Giant-Gecko/) (Cortex-M3)
+* [EFM32 Wonder Gecko] (https://developer.mbed.org/platforms/EFM32-Wonder-Gecko/) (Cortex-M4)
+
 
 Supported Toolchains and IDEs
 -----------------------------