Add 5.5 migration guide and notes for cakephp/cakephp#19584

Author: markstory

docs/en/appendices/5-5-migration-guide.md

@@ -0,0 +1,30 @@
+# 5.5 Migration Guide
+
+The 5.5.0 release is backwards compatible with 5.0. It adds new functionality
+and introduces new deprecations. Any functionality deprecated in 5.x will be
+removed in 6.0.0.
+
+## Upgrade Tool
+
+The [upgrade tool](../appendices/migration-guides) provides rector rules for
+automating some of the migration work. Run rector before updating your
+`composer.json` dependencies:
+
+```text
+bin/cake upgrade rector --rules cakephp55 <path/to/app/src>
+```
+
+## Behavior Changes
+
+### Database
+
+- Expressions created with `FunctionsBuilder::concat()` now use `CONCAT()` with
+  postgres instead of the `||` operator.
+
+## Deprecations
+
+Coming soon
+
+## New Features
+
+Coming soon

docs/en/appendices/migration-guides.md

@@ -37,4 +37,5 @@ to ensure the tool can resolve class names correctly.
 - [5.2 Migration Guide](5-2-migration-guide)
 - [5.3 Migration Guide](5-3-migration-guide)
 - [5.4 Migration Guide](5-4-migration-guide)
+- [5.5 Migration Guide](5-5-migration-guide)
 - [PHPUnit Upgrade](phpunit-upgrade)