add migration instructions for safemem (#1909)

- as suggested in <#1615 (comment)>

Author: Skgland

crates/safemem/RUSTSEC-2023-0081.md

@@ -13,3 +13,17 @@ patched = []
 # safemem is unmaintained
 
 The latest crates.io release was in 2019. The repository has been archived by the author.
+
+## Migration
+
+- `safemem::copy_over(slice, src_idx, dest_idx, len);` can be replaced with `slice.copy_within(src_idx..src_idx+len, dest_idx);` as of `rust 1.37.0`.
+- `safemem::write_bytes(slice, byte);` can be replaced with `slice.fill(byte);` as of `rust 1.50.0`
+- `safemem::prepend(slice, vec);` can be replaced with
+
+  ```rust
+  let old_len = vec.len();
+  vec.extend_from_slice(slice);
+  vec.rotate_left(old_len);
+  ```
+  
+  as of `rust 1.26.0`