Bring back UPGRADE.md for now

Author: mattstauffer

UPGRADE.md

@@ -0,0 +1,12 @@
+# Upgrading to v4
+
+- You must have PHP 8.0+ installed, and set as your primary/linked PHP install when you install v4; you can use 7.4+ as your primary after that, but need to keep a version of 8+ installed at all times
+- You must run `valet` once for the upgrader to run; this will attempt to update all of your configuration items and custom local drivers for v4
+- If you have any issues with your drivers, all custom drivers (including the `SampleValetDriver` published by previous versions of Valet) must have the following; see the [new SampleValetDriver](https://github.com/laravel/valet/blob/d7787c025e60abc24a5195dc7d4c5c6f2d984339/cli/stubs/SampleValetDriver.php) for an example:
+    - Match the new type hints of the base ValetDriver
+    - Extend the new namespaced drivers instead of the old globally-namespaced drivers
+    - Have their own namespace
+- If you have any `.valetphprc` files in your projects, update them to use the new `.valetrc` syntax:
+    - Rename `.valetphprc` to `.valetrc`
+    - Add `php=` before the version number; e.g. `[email protected]`
+- If you're using Expose and want to connect it to Valet, make sure to upgrade to v2.6.0 or higher