Add information to Float

The current reference manual does not clearly show that Float() can take hexadecimalfloating point format as an argument.
According to https://github.com/ruby/ruby/tree/v1_9_2_381, Float() has supported hexadecimal floating point format since Ruby 1.9.2.
The reference manual of 1.9.2 and later versions will show the exmaple with an argument of hexadecimal floating format.
Merge pull request #51 from monkey-mas/add-infomation-to-Float

Add information to Float