Add migration guide note

MIGRATION_TO_V3.md

@@ -0,0 +1,53 @@
+# Migration Guide to v3.0.0
+
+There were a few breaking changes introduced in `v3.0.0`.  This guide will
+document the changes and what you will need to fix to make the integration work
+if you previously had an older version installed.
+
+## Configuration
+
+Before `v3.0.0` configuration was done as a platform.  This was changed to now
+simply be a top level configuration in your `configuration.yaml`.  See the example
+below of the old configuration and what it should look like in `v3.0.0`.
+
+```yaml
+# Old config prior to v3.0.0
+sensor:
+- platform: nintendo_wishlist
+  country: US
+  wishlist:
+    - Katana ZERO
+    - OKAMI HD
+    - Salt and Sanctuary
+    - Dead Cells
+    - Bloodstained
+    - Dark Souls
+    - Velocity X
+    - "The Legend of Zelda: Breath of the Wild"
+
+# New config in v3.0.0
+nintendo_wishlist:
+  country: US
+  wishlist:
+    - Katana ZERO
+    - OKAMI HD
+    - Salt and Sanctuary
+    - Dead Cells
+    - Bloodstained
+    - Dark Souls
+    - Velocity X
+    - "The Legend of Zelda: Breath of the Wild"
+```
+
+## Sensors
+
+Prior to `v3.0.0` a `sensor` was created for each game in your wishlist.  It's
+state would be `0` if it was not on sale and `1` if it was on sale.  Now instead
+of a `sensors`, each game on your wishlist will create a `binary_sensor`.  The
+state will be `true` if the game is on sale and `false` if it is not on sale.
+
+### Attributes
+
+The attribute `on_sale` for each `binary_sensor` on your wishlist was renamed to
+`matches`.  If you have automations using the old `on_sale` attribute, you will
+need to rename it to `matches`.

README.md

@@ -4,7 +4,9 @@
 [![hacs_badge](https://img.shields.io/badge/HACS-Default-orange.svg?style=for-the-badge)](https://github.com/custom-components/hacs)
 [![](https://img.shields.io/github/license/custom-components/sensor.nintendo_wishlist?style=for-the-badge)](LICENSE)
 
-#### NOTE: This component currently only works in certain countries.  See [Supported Countries](#supported-countries) below.
+> **_NOTE:_** This component currently only works in certain countries.  See [Supported Countries](#supported-countries) below.
+
+> **_Migrating to v3?_** See the [migration guide](./MIGRATION_TO_V3.md).
 
 Home Assistant component that keeps track of Nintendo Switch games that are on
 sale on your wish list in home assistant.