Document available options

[wontheone1]

As we expected swagger1st to allow undefined properties in definitions, but it didn't, it took me a while to find out actually we can allow this using an option(:allow-undefined-keys) in parser, like below. (s1st/parser :allow-undefined-keys true) This and if there are other undocumented options, it should be documented so that devs can easily discover the option and use them.