Added thermistor table for the 500C thermistor shipped with the Pico hotend

Added the thermistor table for the 500C thermistor that is shipping with the [Pico hot end](http://www.b3innovations.com/?product=pico-ks). I've just taken B3 Innovations supplied [temperature table](http://www.b3innovations.com/wp-content/uploads/2014/04/500c-Temp-Table.txt).

I'm not sure how the thermistor constants are assigned; I just arbitrarily chose 70. Hopefully that's OK.