Add documentation for `as_enum` `:strings` option to README

I noticed that the `strings` option for `as_enum` wasn't mentioned in the README (I only found it myself while browsing the code :) ), thought it was worth adding.

Additionally, I can't seem to find a reference to `strings` in the the PR for 2.0 (https://github.com/lwe/simple_enum/pull/36), it looks like this feature is still supported, has the name of the option changed, or is it inferred?