Add back the E0066 error code long explanation

Author: GuillaumeGomez

src/librustc_error_codes/error_codes/E0066.md

@@ -0,0 +1,9 @@
+#### Note: this error code is no longer emitted by the compiler.
+
+Box placement expressions (like C++'s "placement new") do not yet support any
+place expression except the exchange heap (i.e. `std::boxed::HEAP`).
+Furthermore, the syntax is changing to use `in` instead of `box`. See [RFC 470]
+and [RFC 809] for more details.
+
+[RFC 470]: https://github.com/rust-lang/rfcs/pull/470
+[RFC 809]: https://github.com/rust-lang/rfcs/pull/809