Update migrating guide for v16.0.0 (#2694)

MIGRATING.md

@@ -1,5 +1,22 @@
 # Migration Guide
 
+## Migrating from versions < 16.0.0
+- Changes to `CardInputWidget` and `CardMultilineWidget`
+    - `card` and `cardBuilder` are deprecated. Use `cardParams` instead.
+- Changes to `Card`
+    - `number` and `cvc` are deprecated and will be removed in a future release.
+    - `Card.Builder` is deprecated. Use `CardParams` instead.
+    - `Card.fromString()`, `Card.fromJson()`, and `Card.create()` are deprecated
+      and will be removed in a future release.
+- Changes to `SourceParams`
+    - `createCardToken()` that accepts a `Card` parameter is deprecated.
+      Use `createCardToken()` that accepts a `CardParams` parameter instead.
+    - `createCardTokenSynchronous()` that accepts a `Card` parameter is deprecated.
+      Use `createCardTokenSynchronous()` that accepts a `CardParams` parameter instead.
+- Changes to `Stripe`
+    - `createCardParams()` that accepts a `Card` parameter is deprecated.
+      Use `createCardParams()` that accepts a `CardParams` parameter instead.
+
 ## Migrating from versions < 15.0.0
 - The SDK now targets JVM 1.8
 - The SDK now requires Android 5.0+ (API level 21+)