added MIGRATION.md

Author: avigoldman

MIGRATION.md

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+# Migration Guide
+
+This is a guide to help you make the switch when the SparkPost PHP library changes major versions.
+
+## Migrating from 1.0 to 2.0
+
+## Package name change
+The composer package name has changed from `sparkpost/php-sparkpost` to `sparkpost/sparkpost`
+
+### No more setupUnwrapped
+We replaced the idea of 'wrapping' API resources with a simple `request` function. To see it in action, check out this [example](https://github.com/SparkPost/php-sparkpost/tree/2.0.0#send-an-api-call-using-the-base-request-function).
+
+### `transmission` becomes `transmissions`
+Transmission endpoints are now under `$sparky->transmissions` instead of `$sparky->transmission` to map more directly to the [API docs](https://developers.sparkpost.com/api/).
+
+* We no longer map parameters to the API - we simplified. Instead custom mapping, now set the payload to match the API docs.
+* The exceptions to the previous statement are `cc` and `bcc`. They are helpers to make it easier to add cc and bcc recipients. [Example](https://github.com/SparkPost/php-sparkpost/tree/2.0.0#send-an-email-using-the-transmissions-endpoint)
+
+### Switched from Ivory Http Adapter to HTTPlug
+Ivory Http Adapter was deprecated in favor fo HTTPlug.
+
+### Asynchronous support
+We addeded in support for [asynchronous calls](https://github.com/SparkPost/php-sparkpost/tree/2.0.0#asynchronous) (assuming your client supports it).