CHANGELOG: add some headings for better readability

Author: jtdaugherty

CHANGELOG.md

@@ -10,6 +10,8 @@ applications that want to support multiple Vty platforms at build time,
 and at the time of this release, a release of a `vty-windows` package is
 forthcoming.
 
+**Migration guide for 6.0**
+
 To upgrade to this version of Vty, most people will only need to take a
 few steps:
 
@@ -33,6 +35,8 @@ initialization and rendering API, the full change list is provided
 below. For people who want to write their own Vty platform package like
 `vty-unix`, see `PLATFORM-HOWTO.md`.
 
+**Detailed change list for 6.0**
+
 * Package changes:
   * The following modules got added to the `vty` library:
     * `Graphics.Vty.UnicodeWidthTable.Main`