chore: add migration doc

Author: wemeetagain

doc/migrations/v0.46-v1.0.md

@@ -46,7 +46,7 @@ const libp2p = await createLibp2p(...)
 const keychain: KeyChain = libp2p.keychain
 ```
 
-***After***
+**After**
 
 ```ts
 import { DefaultKeyChain } from '@libp2p/keychain'
@@ -64,6 +64,22 @@ const libp2p = await createLibp2p({
 const keychain: KeyChain = libp2p.services.keychain
 ```
 
+### AutoNAT
+
+The AutoNAT service is now published in its own package.
+
+**Before**
+
+```ts
+import { autoNATService } from 'libp2p/autonat'
+```
+
+**After**
+
+```ts
+import { autoNATService } from '@libp2p/autonat'
+```
+
 ## Module Updates
 
 With this release you should update the following libp2p modules if you are relying on them: