explain what the open questions are, and add a Miri test for that

Author: RalfJung

tests/pass/drop_in_place.rs

@@ -0,0 +1,12 @@
+// Miri currently doesn't require types without drop glue to be
+// valid when dropped. This test confirms that behavior.
+// This is not a stable guarantee!
+
+use std::ptr;
+
+fn main() {
+    let mut not_a_bool = 13u8;
+    unsafe {
+        ptr::drop_in_place(&mut not_a_bool as *mut u8 as *mut bool)
+    };
+}