`README.md` | Add note about cache busting for files, `pipeline.AddFiles()`.

[salembream]

Hello there, I recently wanted to use WebOptimizer for a project I work on, cache busting didn't work for me until I figured out (by debugging the library codes) that the file should be added somehow to the assets.

If this note (or at least giving a hint about pipeline.AddFiles() function) was in the README it would save me a lot of time, hope it will do for the others.