run-make-support: add some top-level docs

src/tools/run-make-support/src/lib.rs

@@ -1,3 +1,8 @@
+//! `run-make-support` is a support library for run-make tests. It provides command wrappers and
+//! convenience utility functions to help test writers reduce duplication. The support library
+//! notably is built via cargo: this means that if your test wants some non-trivial utility, such
+//! as `object` or `wasmparser`, they can be re-exported and be made available through this library.
+
 pub mod cc;
 pub mod run;
 pub mod rustc;