Add a comment for the outdated crossbeam-deque

ref: rayon-rs#480 (comment)

Author: cuviper

rayon-core/Cargo.toml

@@ -16,8 +16,12 @@ categories = ["concurrency"]
 [dependencies]
 rand = ">= 0.3, < 0.5"
 num_cpus = "1.2"
-crossbeam-deque = "0.2.0"
 libc = "0.2.16"
 lazy_static = "1"
 
+# This is deliberately not the latest version, because we want
+# to support older rustc than crossbeam-deque 0.3+ does.
+[dependencies.crossbeam-deque]
+version = "0.2.0"
+
 [dev-dependencies]