rust: proc-macro2: add README.md

Originally, when the Rust upstream `alloc` standard library crate was
vendored in commit 057b8d2 ("rust: adapt `alloc` crate to the
kernel"), a `README.md` file was added to explain the provenance and
licensing of the source files.

Thus do the same for the `proc-macro2` crate.

Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Author: ojeda

rust/proc-macro2/README.md

@@ -0,0 +1,13 @@
+# `proc-macro2`
+
+These source files come from the Rust `proc-macro2` crate, version
+1.0.101 (released 2025-08-16), hosted in the
+<https://github.com/dtolnay/proc-macro2> repository, licensed under
+"Apache-2.0 OR MIT" and only modified to add the SPDX license
+identifiers and to remove the `unicode-ident` dependency.
+
+For copyright details, please see:
+
+    https://github.com/dtolnay/proc-macro2/blob/1.0.101/README.md#license
+    https://github.com/dtolnay/proc-macro2/blob/1.0.101/LICENSE-APACHE
+    https://github.com/dtolnay/proc-macro2/blob/1.0.101/LICENSE-MIT