Add some docs about cross-compilation

README.md

@@ -14,6 +14,7 @@ Features:
 * Receive notifications of updates to release channels.
 * Install historical nightly toolchains from the official archives.
 * Install by specific stable version number.
+* Install additional `std`s for cross-compilation.
 * Install custom toolchains.
 * Isolate Cargo metadata per-installation.
 * Verify hashes of downloads.
@@ -126,6 +127,13 @@ the archive for that date (if available). Any other specifier is
 considered an explicit Rust version number, as in '0.12.0', or a custom
 toolchain identifier, depending on context.
 
+# Cross-compilation
+
+`multirust` can install additional standard libraries for
+cross-compilation targets. Once you have installed a toolchain, use
+the `multirust list-available-targets` command to show which targets
+can be added, and `multirust add-target` to add a target.
+
 # Custom toolchains
 
 Custom toolchains - those not distributed by The Rust Project - may be