docs: add upgrade doc

documentation/how_to/upgrade.md

@@ -0,0 +1,9 @@
+# Upgrading from 2.0 to 3.0
+
+There are a few major changes in 3.0 that you should be aware of that affect AshPhoenix's behavior.
+
+1. You don't need to pass in the `domain` to your forms.
+
+2. Extra inputs are no longer accepted by actions. This means that if you were using custom form parameters, you may run into issues. There is a new option when constructing forms, `skip_unknown_inputs: ["foo", "bar"]` that allows you to skip these, but they don't currently apply to nested forms. If you need this, or encounter any issues, please open an issue.
+
+3. When calling `AshPhoenix.Form.params/1`, for the same reasons as above, it will no longer return the hidden fields from the form. If you need these, you can add the `hidden?: true` option to `AshPhoenix.Form.params/1`.

mix.exs

@@ -61,7 +61,8 @@ defmodule AshPhoenix.MixProject do
       logo: "logos/small-logo.png",
       extras: [
         "documentation/tutorials/getting-started-with-ash-and-phoenix.md",
-        "documentation/topics/working-with-phoenix.md"
+        "documentation/topics/working-with-phoenix.md",
+        "documentation/how_to/upgrade.md"
       ],
       groups_for_extras: [
         Tutorials: ~r'documentation/tutorials',