Add docs for -Zmiri-check-number-validity

Author: camelid

README.md

@@ -199,6 +199,10 @@ up the sysroot.  If you are using `miri` (the Miri driver) directly, see the
 Miri adds its own set of `-Z` flags, which are usually set via the `MIRIFLAGS`
 environment variable:
 
+* `-Zmiri-check-number-validity` enables checking of integer and float validity
+  (e.g., they must be initialized and not carry pointer provenance) as part of
+  enforcing validity invariants. This has no effect when
+  `-Zmiri-disable-validation` is present.
 * `-Zmiri-compare-exchange-weak-failure-rate=<rate>` changes the failure rate of
   `compare_exchange_weak` operations. The default is `0.8` (so 4 out of 5 weak ops will fail).
   You can change it to any value between `0.0` and `1.0`, where `1.0` means it