Add table-of-contents link to the hint-mostly-unused section

Author: joshtriplett

src/doc/src/reference/unstable.md

@@ -101,6 +101,7 @@ Each new feature described below should explain how to use it.
     * [rustdoc-depinfo](#rustdoc-depinfo) --- Use dep-info files in rustdoc rebuild detection.
 * `Cargo.toml` extensions
     * [Profile `rustflags` option](#profile-rustflags-option) --- Passed directly to rustc.
+    * [Profile `hint-mostly-unused` option](#profile-hint-mostly-unused-option) --- Hint that a dependency is mostly unused, to optimize compilation time.
     * [codegen-backend](#codegen-backend) --- Select the codegen backend used by rustc.
     * [per-package-target](#per-package-target) --- Sets the `--target` to use for each individual package.
     * [artifact dependencies](#artifact-dependencies) --- Allow build artifacts to be included into other build artifacts and build them for different targets.