Add UPGRADE-4.0 stub

.travis.yml

@@ -7,9 +7,9 @@ cache:
     - $HOME/.composer/cache
 
 php:
-  - 5.5
   - 5.6
   - 7.0
+  - 7.1
   - nightly
   - hhvm
 

UPGRADE-4.0.md

@@ -0,0 +1,11 @@
+Upgrading from PHP-Parser 3.x to 4.0
+====================================
+
+### PHP version requirements
+
+PHP-Parser now requires PHP 5.6 or newer to run. It is however still possible to *parse* PHP 5.2-5.5
+source code, while running on a newer version.
+
+### Removed functionality
+
+* Removed `type` subnode on `Class`, `ClassMethod` and `Property` nodes. Use `flags` instead.