Users should be warned of Apache/GPLv2 compatibility concerns

Choosing to use the Apache license instead of MIT means that some "GPLv2 only" projects may choose not to use the software. This is rarely something that users of a permissive license actually want.

While the legal situation is murky (the ASF and FSF disagree about whether the licenses are actually incompatible), this is not just a theoretical problem from a social perspective: https://github.com/twitter/bootstrap/issues/2054