changelog: Add back "unreleased changes" section

bgamari · bgamari · commit 758d2f799020 · 2020-03-19T22:45:30.000-04:00
diff --git a/changelog.md b/changelog.md
@@ -1,5 +1,7 @@
 # Changelog for [`process` package](http://hackage.haskell.org/package/process)
 
+## Unreleased changes
+
 ## 1.6.8.2 *March 2020*
 
 * Fix another process wait bug on Windows.
