Added more GPIO pins (incl. B+) and explanation

Untested. Wiring Pi numbers taken from source [1].

[1] https://git.drogon.net/?p=wiringPi;a=blob;f=wiringPi/wiringPi.c;h=e7ae3863c7ca0fe6f5b66e08a833293965d9325b;hb=df45388f6431f7baba31ac1e8e242d89828637a0
Merge pull request #108 from tjanson/gpio-more-pins

Added more GPIO pins (incl. B+) and explanation