Update CHANGELOG (#6399)

## Motivation

Update CHANGELOG for `v1.7.6`.

CHANGELOG.md

@@ -2,6 +2,20 @@
 
 See [RELEASE](./RELEASE.md) for workflow instructions.
 
+## v1.7.6
+
+### Upgrade information
+
+The previous version of go-spacemesh introduced a bug into the code that handles incoming ATXs. This bug would cause
+the node to store invalid data in the database and could lead to a node disagreeing about the state of the network with
+other nodes. This version fixes the bug. If you are running v1.7.5 you should upgrade to v1.7.6 as soon as possible and
+rebuild your `state.db`, either via quicksync, from a backup from before you upgraded or by full syncing from genesis.
+
+### Improvements
+
+* [#6398](https://github.com/spacemeshos/go-spacemesh/pull/6398) Fixed a bug where the node would store invalid
+  information about ATXs in the database.
+
 ## v1.7.5
 
 ### Upgrade information