chore: set union merge strategy to changelog.md (ignite#2814)

Since we are forcing the updage of changelog.md on every PR, this leads
to many conflicts when it's time to merge.

By forcing the merge strategy of this file to `union`, it decreases the
chance of conflicts by ~90%. The downside is sometimes the merge gets
wrong (repeated lines) so the dev should have a look at the result.

Author: tbruyelle

.gitattributes

@@ -0,0 +1 @@
+changelog.md merge=union

changelog.md

@@ -14,6 +14,7 @@
 
 ### Changes
 
+- Add changelog merge strategy in .gitattributes to avoid conflicts.
 - Refactor `templates/app` to remove `monitoringp` module from the default template
 - Updated keyring dependency to match what Cosmos SDK is using
 - Speed up the integration tests