Explicitly mention compiletest directives are supported in rmake.rs (rust-lang#1949)

Author: jieyouxu

src/tests/compiletest.md

@@ -396,6 +396,9 @@ with the `run_make_support` library linked in.
 If you need new utilities or functionality, consider extending and improving
 the [`run_make_support`] library.
 
+Compiletest directives like `//@ only-<target>` or `//@ ignore-<target>` are supported in
+`rmake.rs`, like in UI tests.
+
 Two `run-make` tests are ported over to Rust recipes as examples:
 
 - <https://github.com/rust-lang/rust/tree/master/tests/run-make/CURRENT_RUSTC_VERSION>