docs: add prelude note to migration

docs/docs/misc/migration_notes.md

@@ -8,6 +8,24 @@ Aztec is in full-speed development. Literally every version breaks compatibility
 
 ## 0.25.0
 
+### [Aztec.nr] Introduction to `prelude`
+
+A new `prelude` module to include common Aztec modules and types.
+This simplifies dependency syntax. For example:
+```rust
+use dep::aztec::protocol_types::address::AztecAddress;
+    use dep::aztec::{
+        context::{PrivateContext, Context}, note::{note_header::NoteHeader, utils as note_utils},
+        state_vars::Map
+    };
+```
+Becomes:
+```rust
+    use dep::aztec::prelude::{AztecAddress, Map};
+    use dep::aztec::context::Context;
+```
+This will be further simplified in future versions (See [4496](https://github.com/AztecProtocol/aztec-packages/pull/4496) for further details).
+
 ### [Aztec.nr] No SafeU120 anymore!
 Noir now have overflow checks by default. So we don't need SafeU120 like libraries anymore.