explain arument passing in the docs

Author: RalfJung

README.md

@@ -48,6 +48,13 @@ Now you can run your project in Miri:
 3. If you have a binary project, you can run it through Miri using `cargo
    +nightly miri run`.
 
+You can pass arguments to Miri after the first `--`, and pass arguments to the
+interpreted program or test suite after the second `--`.  For example, `cargo
++nightly miri run -- -Zmiri-disable-validation` runs the program without
+validation of basic type invariants and references.  `cargo +nightly miri test
+-- -- filter` passes `filter` to the test suite the same way `cargo test filter`
+would.
+
 When running code via `cargo miri`, the `miri` config flag is set.  You can
 use this to exclude test cases that will fail under Miri because they do things
 Miri does not support: