Missing documentation for successResponses

[vascofg]

I just spent a lot of time trying to find a way to override the response codes that are cached only to find out that you already support this through the successResponses option.
Is there any reason for this not being documented in the README?

Should I submit a pull request?

[mbj36]

@vascofg This library is more of a deprecated one, Please move to https://github.com/GoogleChrome/workbox