[licenses] Add path to LICENSE file in README.fuchsia.

This makes it easier for license checking tools to find
the correct license file.

More info: http://shortn/_S8Rzw4fSCw

Change-Id: I583e96bcdebde7335b3dfdc68bb4f9d19e6a81f7
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/tink/+/767683
Reviewed-by: Yuxiao Sun <yuxiaosun@google.com>
Commit-Queue: Jerry Belton <jcecil@google.com>