Update Readme.

Depends on #4.
This shows me that this is a bit problematic. I know we discussed it once or more times, but `filePath` seems wrong to me.
I see we now allow passing either json object or filePath, but that's quite confusing. Let's keep *single signature* constructor and allow only json object. Let's replace instance method `Config#parse(filePath)` with static method `Config::createFromConfigFile(filePath)`.