rust: alloc: mention license explicitly in README

Link: https://lore.kernel.org/lkml/CAHk-=wh00UW9i+hGYjmLZW5MOAti9FFBarGBL909k5PfH4r2fg@mail.gmail.com/
Link: https://lore.kernel.org/lkml/CANiq72nhKFFVHA9ZAfWw7=RTTcp_TUsgh8ENGsF8Gn_Qi=D2CQ@mail.gmail.com/
Signed-off-by: Miguel Ojeda <[email protected]>

Author: ojeda

rust/alloc/README.md

@@ -1,8 +1,9 @@
 # `alloc`
 
 These source files come from the Rust standard library, hosted in
-the https://github.com/rust-lang/rust repository. For copyright
-details, see https://github.com/rust-lang/rust/blob/master/COPYRIGHT.
+the https://github.com/rust-lang/rust repository, licensed under
+"Apache-2.0 OR MIT" and adapted for kernel use. For copyright details,
+see https://github.com/rust-lang/rust/blob/master/COPYRIGHT.
 
 Please note that these files should be kept as close as possible to
 upstream. In general, only additions should be performed (e.g. new