Add Unstable Book entry for 'overlapping-marker-traits'.

Author: frewsxcv

Diff for: src/doc/unstable-book/src/SUMMARY.md

@@ -136,6 +136,7 @@
 - [optin_builtin_traits](optin-builtin-traits.md)
 - [option_entry](option-entry.md)
 - [osstring_shrink_to_fit](osstring-shrink-to-fit.md)
+- [overlapping_marker_traits](overlapping-marker-traits.md)
 - [panic_abort](panic-abort.md)
 - [panic_runtime](panic-runtime.md)
 - [panic_unwind](panic-unwind.md)

Diff for: src/doc/unstable-book/src/overlapping-marker-traits.md

@@ -0,0 +1,7 @@
+# `overlapping_marker_traits`
+
+The tracking issue for this feature is: [#29864]
+
+[#29864]: https://github.com/rust-lang/rust/issues/29864
+
+------------------------