Update README.md

Remark about the AssetFactory's rootDir
Explain what workers are

I found it confusing as workers are mentioned in the "Cache Busting" section but it is not mentioned before what they are.
A bit more information about what the target path is and what CacheBustingWorker does
Reorder sections

I was at first confused what "Cache busting" and "Caching" have in common, but if I am not mistaken, they are for different purposes.

I hope this ordering makes that more clear and helps to understand what cache busting is good for.
CacheBustingWorker no longer provides two strategies and has changed the constructor's signature.

See https://github.com/kriswallsmith/assetic/pull/362#commitcomment-3208549
Merge pull request #428 from mpdude/patch-1

Update README.md