More makefile docs

docs/rust.rst

@@ -5,6 +5,31 @@ Euler Rust Implementation
     :start-line: 2
     :end-before: Problems Solved
 
+Usage
+-----
+
+.. highlight:: make
+
+This folder contains a Makefile with several recipes, most of which are aliases to ``cargo`` commands.
+This facilitates the root Makefile dispatching tasks to each language, many of which have more complex
+build or test processes.
+
+.. make:target:: test
+
+  Alias for ``cargo test``.
+
+.. make:target:: test_auto
+
+  Runs tests in parallel with one less thread than you have CPUs. Alias for ``cargo test -j``.
+
+.. make:target:: test_%
+
+  Runs tests in parallel with the specified number of threads. Alias for ``cargo test -j $*``.
+
+.. make:target:: clean
+
+  Alias for ``cargo clean``.
+
 Problems Solved
 ---------------