Add additional note about -Zmiri-panic-on-unsupported

Author: landaire

README.md

@@ -231,7 +231,8 @@ environment variable:
 * `-Zmiri-panic-on-unsupported` will makes some forms of unsupported functionality
   such as FFI and unsupported syscalls panic within the context of the emulated
   application instead of raising an error within the context of Miri (and halting
-  execution).
+  execution). Note that code might not expect these operations to ever panic, so
+  this flag can lead to strange(mis)behavior.
 * `-Zmiri-seed=<hex>` configures the seed of the RNG that Miri uses to resolve
   non-determinism.  This RNG is used to pick base addresses for allocations.
   When isolation is enabled (the default), this is also used to emulate system