message: Update readme to make supported codo clearer
diff: @@ -100,6 +100,30 @@ fn does_not_work_on_miri() {
  }
  ```
  +An exhaustive list of what `miri` does not support is not available, as this could be
  +an unbounded set with FFI and more. However `miri` will explicitly tell you when it finds
  +something unsupported with an error:
  +
  +```
  +error: unsupported operation: can't call foreign function: mach_timebase_info
  + --> /home/r/.rustup/toolchains/miri/lib/rustlib/src/rust/src/libstd/sys/unix/time.rs:239:13
  + |
  +239 | mach_timebase_info(&mut info);
  + | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ can't call foreign function: mach_timebase_info
  + |
  + = help: this is likely not a bug in the program; it indicates that the program performed an operation that the interpreter does not support
  + = note: inside call to `std::sys::unix::time::inner::info` at /home/r/.rustup/toolchains/miri/lib/rustlib/src/rust/src/libstd/sys/unix/time.rs:156:24
  + = note: inside call to `std::sys::unix::time::inner::Instant::checked_sub_instant` at /home/r/.rustup/toolchains/miri/lib/rustlib/src/rust/src/libstd/time.rs:263:9
  +note: inside call to `std::time::Instant::duration_since` at tests/run-pass/time.rs:25:20
  + --> tests/run-pass/time.rs:25:20
  + |
  +25 | let diff = now2.duration_since(now1);
  + | ^^^^^^^^^^^^^^^^^^^^^^^^^
  + = note: inside call to `main` at /home/r/.rustup/toolchains/miri/lib/rustlib/src/rust/src/libstd/rt.rs:67:34
  +```
  +
  +If you do not see an error like this, you are able to keep using `miri`!
  +
  ### Running Miri on CI
  To run Miri on CI, make sure that you handle the case where the latest nightly
  