Clarify permission to publish project using Apache v2 license

[bbottema]

The project this library originally continued was published under GPL3, which is not forward compatible with Apache v2, unless you have explicit permission to do so.

Now, this library has basically evolved into a complete rewrite, so it remains to be seen in how far a permission is still required, but this is a bit of a grey area. Better to be safe and clear on it for 3rd party users.

[bbottema]

I have contacted the original developer and obtained express permission to continue the project under Apache v2. Details can be found in the NOTICE file.