Add unmaintained crate advisory for stdweb

Closes #391

Author: tarcieri

crates/stdweb/RUSTSEC-0000-0000.md

@@ -0,0 +1,22 @@
+```toml
+[advisory]
+id = "RUSTSEC-0000-0000"
+package = "stdweb"
+date = "2020-05-04"
+url = "https://github.com/koute/stdweb/issues/403"
+informational = "unmaintained"
+
+[versions]
+patched = []
+unaffected = ["> 0.4.20"]
+```
+
+# stdweb is unmaintained
+
+The author of the `stdweb` crate is unresponsive.
+
+Maintained alternatives:
+
+- [`wasm-bindgen`](https://github.com/rustwasm/wasm-bindgen)
+- [`js-sys`](https://github.com/rustwasm/wasm-bindgen/tree/master/crates/js-sys)
+- [`web-sys`](https://github.com/rustwasm/wasm-bindgen/tree/master/crates/web-sys)