Enhancement to documentation

[mrturck]

Hi I am new to using this library. Managed to figure things out from youtube tutorials, but wanted to add what I thought was missing from the current documentation:

• For beginners: linking resources on how to obtain a private key and add funds if they have never used datapay or moneybutton before.
• For developers: more examples of the send callback function so devs can see what type of data they are receiving

[mrturck]

Additionally any references pointing to a list of potential error codes and explanations would be very useful.