Change README advice to use +=

Since NonStupidDigestAssets.whitelist is an empty array, the array
can be added to rather than replaced. This better enables gem authors
to depend on this gem and add their own assets to the whitelist.
Merge pull request #23 from adamonduty/readme-fix

Change README advice to use +=
Merge github.com:alexspeller/non-stupid-digest-assets into issue-#26