Add more context in Introduction about efficient rejection of malformed inputs

[fxamacker]

Updates #92

Update "Introduction" section to add:

CCF uses CBOR and is designed to allow efficient detection and rejection of malformed messages without creating Cadence objects. This allows more costly checks for validity, etc. to be performed only on well-formed messages.

This aspect of CCF and CBOR helps reject malformed inputs quickly and with less memory.