Add advisory for data race in thex (#523)

crates/thex/RUSTSEC-0000-0000.md

@@ -0,0 +1,19 @@
+```toml
+[advisory]
+id = "RUSTSEC-0000-0000"
+package = "thex"
+date = "2020-12-08"
+categories = ["memory-corruption"]
+keywords = ["concurrency"]
+
+[versions]
+patched = []
+```
+
+# Thex<T> allows data races of non-Send types across threads
+
+`thex::Thex<T>` implements `Sync` for all types `T`. However, it is missing a
+bound for `T: Send`.
+
+This allows non-Send types such as `Rc` to be sent across thread boundaries
+which can trigger undefined behavior and memory corruption.