Add a top-level LICENSE file

We are hoping to import the GBL repo into the Fuchsia source tree to
reuse libraries such as A/B where the bootloader and OS need to be
consistent with each other. A LICENSE file is needed at the top-level
before doing so.

Bug: 336327811
Change-Id: Ibbe6df96939921fe6a2b30756b8317b6263c2f39