IANAL, But from the readme [1], it's Apache Licensed, so fork away - just make sure you retain their copyright notices and follow the rest of the license.

-edit- In addition, the bottom of the README which you quoted just mentions the need to sign a CLA if you want your code pushed upstream. See the FSF page about why they require copyright assignment [2] for their projects for some background.

[1] http://lcamtuf.coredump.cx/afl/README.txt [2] https://www.gnu.org/licenses/why-assign.html