Fix license issue for angular-sanitize.js file

From "Steve winslow"

"The info you provided is helpful. To address this out of date / ambiguous statement, I'd recommend adding the following to the top-level LICENSE.txt file in clamp:

"For the file /src/main/resources/META-INF/resources/designer/lib/angular-sanitize.js, to the extent that it contains code originating from Erik Arvidsson, that code is used under the Apache-2.0 license, as permitted by http://erik.eae.net/simplehtmlparser/simplehtmlparser.js."

Would you mind adding that line to clamp's LICENSE.txt? I think that would help resolve this issue."