Update documenatation to clarify cacheFile

[jdmunro]

I noticed that .cacheFile doesn't respect the fileDirName that's been set in the options. This means that you might end up downloading the file twice, and can break certain offline solutions. This caught me out for a while so figured it was worth adding to the docs.

A future improvement would be to unify the two so that fileDirName is respected in both cases.

See this line, where no options are passed: https://github.com/billmalarky/react-native-image-cache-hoc/blob/master/lib/imageCacheHoc.js#L60

[coveralls]

Coverage remained the same at 93.548% when pulling f3492bf on jdmunro:master into 20c438a on billmalarky:master.