Add link to Cargo.toml with explanation of packed_simd renaming

(cherry picked from commit 45f3f53)
Signed-off-by: Isis Lovecruft <isis@patternsinthevoid.net>

Cargo.toml

@@ -45,6 +45,8 @@ byteorder = { version = "^1.2.3", default-features = false, features = ["i128"]
 digest = { version = "0.8", default-features = false }
 subtle = { version = "^2.2.1", default-features = false }
 serde = { version = "1.0", default-features = false, optional = true, features = ["derive"] }
+# The original packed_simd package was orphaned, see
+# https://github.com/rust-lang/packed_simd/issues/303#issuecomment-701361161
 packed_simd = { version = "0.3.4", package = "packed_simd_2", features = ["into_bits"], optional = true }
 zeroize = { version = "1", default-features = false }