Add upgrade doc

Author: natewiebe13

UPGRADE-2.0.md

@@ -0,0 +1,14 @@
+# UPGRADE FROM 1.x to 2.0
+
+## PHP version support
+
+- Support for PHP < 7.4 was dropped
+
+## Classes
+
+- `Feather\Icons` was renamed to `Feather\IconManager`
+
+## Icons
+
+- `Feather\Icons::get()` was renamed to `Feather\IconManager::getIcon()`
+- `Feather\Icon` objects are returned by `Feather\IconManager::getIcon()` instead of a `string`