Add upgrade guide

Author: ivanvermeyen

Diff for: README.md

@@ -22,6 +22,11 @@ Automatically detect and set an app locale that matches your visitor's preferenc
 - PHP >= 7.1
 - Laravel >= 5.6
 
+## ⬆️ Upgrade
+
+Upgrading to a new major version?
+Check our [upgrade guide](UPGRADE.md) for instructions.
+
 ## 📦 Install
 
 Install this package with Composer:

Diff for: UPGRADE.md

@@ -0,0 +1,8 @@
+# Upgrade Guide
+
+## Upgrading To 2.0 From 1.x
+
+Every config option that contained a `-` (dash) in its name has been updated and the dash is replaced by an `_` (underscore).
+This is done mainly for consistency across other packages.
+
+Review and update your published config file accordingly.