Add long explanation for E0498

compiler/rustc_error_codes/src/error_codes/E0498.md

@@ -0,0 +1,22 @@
+The `plugin` attribute was malformed.
+
+Erroneous code example:
+
+```compile_fail,E0498
+#![feature(plugin)]
+#![plugin(foo(args))] // error: invalid argument
+#![plugin(bar="test")] // error: invalid argument
+```
+
+The `#[plugin]` attribute should take a single argument: the name of the plugin.
+
+For example, for the plugin `foo`:
+
+```
+#![feature(plugin)]
+#![plugin(foo)] // ok!
+```
+
+See the [`plugin` feature] section of the Unstable book for more details.
+
+[`plugin` feature]: https://doc.rust-lang.org/nightly/unstable-book/language-features/plugin.html