Improved documentation on lcd.char

[4techguns]

When mentioning lcd.char in readme.md, there are absolutely NO instructions on how to set the byte.

I think it would help the users more if you taught them how to set the byte.
The way is to use String.fromCharCode([number]) ranging from 16 through 255. (16 is the first non-blank character)
If someone is using the test.html as a reference guide, just convert the hex character to decimal
An example is lcd.char(0, 0, String.fromCharCode(16));

Hope this helps anyone else who had this problem!