Add a migration guide for 0.7.0

[ci skip]

docs/getting-started/upgrading.md

@@ -6,9 +6,18 @@ If your version of dokku is pre 0.3.0 (check with `dokku version`), we recommend
 
 Before upgrading, check the migration guides to get comfortable with new features and prepare your deployment to be upgraded.
 
+### 0.5 Migration Guide
+
 - [0.5 Migration Guide](/dokku/appendices/0.5.0-migration-guide/)
+
+### 0.6 Migration Guide
+
 - [0.6 Migration Guide](/dokku/appendices/0.6.0-migration-guide/)
 
+### 0.7 Migration Guide
+
+- [0.7 Migration Guide](/dokku/appendices/0.6.0-migration-guide/)
+
 ## Upgrade Instructions
 
 If dokku was installed via `apt-get install dokku` or `bootstrap.sh` (most common), upgrade with: