Add upgrade guide for v2

UPGRADE.md

@@ -1,3 +1,8 @@
+# Upgrade from 1.x to 2.0.0
+
+- Required minimum PHP version is now 8.0.
+- Access to url components via class properties (instead of methods) using magic getter and setter was removed. So if you've been using components like `$url->host`, `$url->path` and so on (see https://www.crwlr.software/packages/url/components), you need to change those to method calls, so just: `$url->host()`, `$url->path()`. The property access was just an alias.
+
 # Upgrade from 0.x to 1.0.0
 
 - Required minimum PHP version is now 7.2.