rust: syn: 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 `syn` crate.

Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Gary Guo <gary@garyguo.net>
Tested-by: Gary Guo <gary@garyguo.net>
Tested-by: Jesung Yang <y.j3ms.n@gmail.com>
Link: https://patch.msgid.link/20251124151837.2184382-19-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Author: ojeda

rust/syn/README.md

@@ -0,0 +1,13 @@
+# `syn`
+
+These source files come from the Rust `syn` crate, version 2.0.106
+(released 2025-08-16), hosted in the <https://github.com/dtolnay/syn>
+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/syn/blob/2.0.106/README.md#license
+    https://github.com/dtolnay/syn/blob/2.0.106/LICENSE-APACHE
+    https://github.com/dtolnay/syn/blob/2.0.106/LICENSE-MIT