Manual.md: document rust build-helper

Manual.md

@@ -24,6 +24,7 @@ packages for XBPS, the `Void Linux` native packaging system.
 		* [Package defined repositories](#pkg_defined_repo)
 	* [Checking for new upstream releases](#updates)
 	* [Build style scripts](#build_scripts)
+	* [Build helper scripts](#build_helper)
 	* [Functions](#functions)
 	* [Build options](#build_options)
 		* [Runtime dependencies](#deps_runtime)
@@ -862,6 +863,17 @@ matching the `build_style` name, Example:
 
     `common/environment/build-style/gnu-configure.sh`
 
+<a id="build_helper"></a>
+### build helper scripts
+
+The `build_helper` variable specifies shell snippets to be sourced that will create a
+suitable environment for working with certain sets of packages.
+
+The current list of available `build_helper` scripts is the following:
+
+- `rust` specifies environment variables required for cross-compiling crates via cargo and
+for compiling cargo -sys crates.
+
 <a id="functions"></a>
 ### Functions