mention new option in README and diagnostics

Author: RalfJung

README.md

@@ -176,6 +176,8 @@ Several `-Z` flags are relevant for Miri:
 * `-Zmiri-disable-stacked-borrows` disables checking the experimental
   [Stacked Borrows] aliasing rules.  This can make Miri run faster, but it also
   means no aliasing violations will be detected.
+* `-Zmiri-disable-alignment-check` disables checking pointer alignment on memory
+  accesses.
 * `-Zmiri-disable-isolation` disables host isolation.  As a consequence,
   the program has access to host resources such as environment variables, file
   systems, and randomness.

src/diagnostics.rs

@@ -97,6 +97,7 @@ pub fn report_error<'tcx, 'mir>(
                     vec![
                         format!("this usually indicates that your program performed an invalid operation and caused Undefined Behavior"),
                         format!("but alignment errors can also be false positives, see https://github.com/rust-lang/miri/issues/1074"),
+                        format!("you can disable the alignment check with `-Zmiri-disable-alignment-check`, but that could hide true bugs")
                     ],
                 UndefinedBehavior(_) =>
                     vec![