Added "NaN is not NaN"

[shreyas-a]

[timruffles]

Great start, but I think currently this isn't thorough enough to be helpful.

First - it could explain why NaN !== NaN: to avoid two nonsensical operations being equal to each other, e.g if(Math.log(-1) === parseInt("%")) { doBadThing() }. Secondly it could mention the isNaN() function, which is the correct way to check for NaN.