Mention skip in README.

Author: cjgillot

tests/mir-opt/README.md

@@ -56,6 +56,8 @@ The LLVM FileCheck tool is used to verify the contents of output MIR against `CH
 present in the test file. This works on the runtime MIR, generated by `--emit=mir`, and not
 on the output of a individual passes.
 
+Use `// skip-filecheck` to prevent FileCheck from running.
+
 To check MIR for function `foo`, start with a `// CHECK-LABEL fn foo(` directive.
 
 `{{regex}}` syntax allows to match `regex`.