Change README to reflect MinSDK 2.3
Change README to reflect MinSDK 2.3

java02014/StickyGridHeaders,bigsui/StickyGridHeaders,silentnuke/StickyGridHeaders,rchandnani/StickyGridHeaders,HiWong/StickyGridHeaders,TonicArtos/StickyGridHeaders,bboyfeiyu/StickyGridHeaders,hgl888/StickyGridHeaders,kopihao/StickyGridHeaders,b2b2244424/StickyGridHeaders,guoxiaojun001/StickyGridHeaders