Update README.md

README.md

@@ -5,6 +5,12 @@ SparkFun Portability Shield
 
 [*SparkFun Portability Shield (DEV-27510)*](https://www.sparkfun.com/products/27510)
 
+The SparkFun Portability Shield adds all the user interface bits needed to take a SparkFun development board into the field. A LiPo battery connector, charger, fuel gauge, and an on/off switch address your power needs. A friction-fit microSD connector allows for data logging. The 1.3in. OLED and a five-way navigation switch provide an excellent user interface. Locking 0.1in. headers along the shield's edge make it easier to solder male pins in place.
+
+I<sup>2</sup>C is used to interface with the display, fuel gauge, and five-way navigation switch (GPIO expander PCA9554). SPI is used to communicate with the microSD card. The power switch uses an ideal diode (LM66200) to disconnect the battery from the system for a sleep current of approximately 4μA.
+
+The Portability Shield has been designed to be usable with any compatible main board, with the [RTK Postcard](https://www.sparkfun.com/products/26916) being the first!
+
 Head to the [Sparkfun Portability Shield Product Manual](http://docs.sparkfun.com/SparkFun_Portability_Shield/) for technical documentation.