chore: add changelog

docs/changelogs/v0.21.md

@@ -6,13 +6,29 @@
 
 - [Overview](#overview)
 - [🔦 Highlights](#-highlights)
+  - [Saving previously seen nodes for later bootstrapping](#saving-previously-seen-nodes-for-later-bootstrapping)
 - [📝 Changelog](#-changelog)
 - [👨‍👩‍👧‍👦 Contributors](#-contributors)
 
 ### Overview
 
 ### 🔦 Highlights
 
+#### Saving previously seen nodes for later bootstrapping
+
+Kubo now stores a subset of connected peers as backup bootstrap nodes ([kubo#8856](https://github.com/ipfs/kubo/pull/8856)).
+These nodes are used in addition to the explicitly defined bootstrappers in the
+[`Bootstrap`](https://github.com/ipfs/kubo/blob/master/docs/config.md#bootstrap) configuration.
+
+This enhancement improves the resiliency of the system, as it eliminates the
+necessity of relying solely on the default bootstrappers operated by Protocol
+Labs for joining the public IPFS swarm. Previously, this level of robustness
+was only available in LAN contexts through [mDNS peer discovery](https://github.com/ipfs/kubo/blob/master/docs/config.md#discoverymdns)
+enabled.
+
+With this update, the same level of robustness is applied to peers that lack
+mDNS peers and solely rely on the public DHT.
+
 ### 📝 Changelog
 
 ### 👨‍👩‍👧‍👦 Contributors