docs(guides): update the migrating guide

Update the title and add small warning about where to look for
future migrations.

Related to #1569

Author: skipjack

src/content/guides/migrating.md

@@ -1,5 +1,5 @@
 ---
-title: Migrating from v1 to v2
+title: Migrating Versions
 sort: 15
 contributors:
   - sokra
@@ -20,6 +20,8 @@ The following sections describe the major changes from webpack 1 to 2.
 
 T> Note that there were far fewer changes between 2 and 3, so that migration shouldn't be too bad. If you are running into issues, please see [the changelog](https://github.com/webpack/webpack/releases) for details.
 
+W> This content may be moved to the blog post in the near future as version 2 has been out for a while. On top of that, version 3 was recently released and version 4 is on the horizon. As noted above, folks should instead to refer to [the changelog](https://github.com/webpack/webpack/releases) for migrations.
+
 
 ## `resolve.root`, `resolve.fallback`, `resolve.modulesDirectories`