84 Update CEP18 Documentation

[ipopescu]

Closes #84

I thought it would be better to merge my two working branches due to some cross-linking.

@deuszex, please help review the following:

• The main contribution to this PR is the cep18 contract README (7080e823f16cf4f94c33ddea7bb2b190fe1ec4a5)
• Additional explanations for the error codes (26f90f3d1290d8bd4cc474fabd1afb1b985f35e6)
• Also, please check the following notes (fb23e91dc7e3f6bb33fa1f535e506b010974b003):
  • the two modalities can not be changed after installation
  • the table listing the events schema
  • the table summarizing the error codes
  • the casper_event_standard crate manages the dictionary storing events

@ACStoneCL, please review the overall documentation updates.

Just FYI, @hoffmannjan and @gyroflaw.

[ipopescu]

Thanks, @ACStoneCL and @deuszex!