Document validation better

[opoudjis]

Not really well documented.

Parsing and validating with a schema (of either kind):

schema = Csvlint::Schema.load_from_json(uri)
validator = Csvlint::Validator.new( "http://example.org/data.csv", nil, schema )

assumes that a schema is online, and doesn't tell you what to do if you have a local schema. Rubydoc isn't any better, have been having to go through source code (and finding bugs).