docs: add license information for pico-universal-flash-nuke module

Signed-off-by: Jos Verlinde <Jos_Verlinde@hotmail.com>

Author: Josverl

mpflash/vendor/readme.md

@@ -10,5 +10,5 @@ These modules are vendored from the following repositories:
 - https://github.com/click-contrib/click-aliases  (Public Domain)
   - click_aliases.py (Robbin Bonthond)
 
-- https://github.com/Gadgetoid/pico-universal-flash-nuke
+- https://github.com/Gadgetoid/pico-universal-flash-nuke (BSD-3-Clause license)
     - universal_flash_nuke.uf2