message: explain which targets we support to what extent
diff: @@ -194,6 +194,28 @@ for seed in $({ echo obase=16; seq 255; } | bc); do
  done
  ```
  +### Supported targets
  +
  +Miri does not support all targets supported by Rust. The good news, however, is
  +that no matter your host OS/platform, it is easy to run code for *any* target
  +using `--target`!
  +
  +The following targets are tested on CI and thus should always work (to the
  +degree documented below):
  +
  +- The best-supported target is `x86_64-unknown-linux-gnu`. Miri releases are
  + blocked on things working with this target. Most other Linux targets should
  + also work well; we do run the test suite on `i686-unknown-linux-gnu` as a
  + 32bit target and `mips64-unknown-linux-gnuabi64` as a big-endian target.
  +- `x86_64-apple-darwin` should work basically as well as Linux. We also test
  + `aarch64-apple-darwin`. However, we might ship Miri with a nightly even when
  + some features on these targets regress.
  +- `x86_64-pc-windows-msvc` works, but supports fewer features than the Linux and
  + Apple targets. For example, file system access and concurrency are not
  + supported on Windows. We also test `i686-pc-windows-msvc`, with the same
  + reduced feature set. We might ship Miri with a nightly even when some features
  + on these targets regress.
  +
  ### Common Problems
  When using the above instructions, you may encounter a number of confusing compiler
  