docs: add peerIdFromString to migration guide (#2781)

Add breaking change in peerIdFromString to migration guide from 1 -> 2

Related #2772

---------

Co-authored-by: Daniel N <[email protected]>
Co-authored-by: Alex Potsides <[email protected]>

Author: 2color

doc/migrations/v1.0.0-v2.0.0.md

@@ -11,6 +11,7 @@ A migration guide for refactoring your application code from libp2p `v1.0.0` to
 - [The `createLibp2p` function accepts a `PrivateKey` instead of a `PeerId`](#the-createlibp2p-function-accepts-a-privatekey-instead-of-a-peerid)
 - [The `@libp2p/keychain` module operates on `PrivateKey`s instead of `PeerId`s](#the-libp2pkeychain-module-operates-on-privatekeys-instead-of-peerids)
 - [The `@libp2p/peer-id-factory` module has been removed](#the-libp2ppeer-id-factory-module-has-been-removed)
+- [`peerIdFromString` no longer accepts CID encoded PeerIDs](#peeridfromstring-no-longer-accepts-cid-encoded-peerids)
 - [The autodialer has been removed](#the-autodialer-has-been-removed)
 - ["Transient" connections have been renamed "limited"](#transient-connections-have-been-renamed-limited)
 - [The CustomEvent polyfill has been removed from `@libp2p/interface`](#the-customevent-polyfill-has-been-removed-from-libp2pinterface)
@@ -251,6 +252,36 @@ const privateKey = await generateKeyPair('Ed25519')
 const peerId = peerIdFromPrivateKey(privateKey)
 ```
 
+## `peerIdFromString` no longer accepts CID encoded PeerIDs
+
+The `peerIdFromString` function used to be quite flexible in that it would attempt
+to interpret multibase strings as either a multihash encoded PeerID or a CID.
+
+As of `@libp2p/[email protected]` the method has been simplified to only accept
+multibase strings containing a multihash encoded PeerID.
+
+To read a PeerId from a string containing a CID, first parse the CID from the
+string, then use the `peerIdFromCID` method.
+
+**Before**
+
+```ts
+import { peerIdFromString } from '@libp2p/peer-id'
+
+const peer = peerIdFromString('k51qzi5uqu5dkwkqm42v9j9kqcam2jiuvloi16g72i4i4amoo2m8u3ol3mqu6s')
+```
+
+**After**
+
+```ts
+import { peerIdFromCID } from '@libp2p/peer-id'
+import { CID } from 'multiformats/cid'
+
+// CID encoded PeerID in base36
+const cid = CID.parse('k51qzi5uqu5dkwkqm42v9j9kqcam2jiuvloi16g72i4i4amoo2m8u3ol3mqu6s')
+const peer = peerIdFromCID(cid)
+```
+
 ## The autodialer has been removed
 
 The libp2p autodialer was a component that attempted to ensure a minimum number