Add marker_trait_attr to the unstable book

Author: scottmcm

src/doc/unstable-book/src/language-features/marker-trait-attr.md

@@ -0,0 +1,33 @@
+# `marker_trait_attr`
+
+The tracking issue for this feature is: [#29864]
+
+[#29864]: https://github.com/rust-lang/rust/issues/29864
+
+------------------------
+
+Normally, Rust keeps you from adding trait implementations that could
+overlap with each other, as it would be ambiguous which to use.  This
+feature, however, carves out an exception to that rule: a trait can
+opt-in to having overlapping implementations, at the cost that those
+implementations are not allowed to override anything (and thus the
+trait itself cannot have any associated items, as they're pointless
+when they'd need to do the same thing for every type anyway).
+
+```rust
+#![feature(marker_trait_attr)]
+
+use std::fmt::{Debug, Display};
+
+#[marker] trait MyMarker {}
+
+impl<T: Debug> MyMarker for T {}
+impl<T: Display> MyMarker for T {}
+
+fn foo<T: MyMarker>(t: T) -> T {
+    t
+}
+```
+
+This is expected to replace the unstable `overlapping_marker_traits`
+feature, which applied to all empty traits (without needing an opt-in).