add initial runtime migration tutorial draft

Author: apopiak

docs/tutorials/runtime-migration/index.md

@@ -0,0 +1,40 @@
+---
+title: Introduction
+---
+
+One of the defining features of the Substrate blockchain development framework is its support for
+**forkless runtime upgrades**. Forkless upgrades are a means of enhancing a blockchain runtime in a
+way that is supported and protected by the capabilities of the blockchain itself. A blockchain's
+[runtime](../../knowledgebase/runtime) defines the [state](../../knowledgebase/runtime/storage) the
+blockchain can hold and also defines the logic for effecting changes to that state. Substrate makes
+it possible to deploy enhanced runtime capabilities (including _breaking_ changes) without a
+[hard fork](../../knowledgebase/getting-started/glossary#fork). These new capabilities expect the
+blockchain's state and storage schema to be configured in the correct way for operation, though.
+If the expected schema and state are not present a [storage migration](../../knowledgebase/runtime/upgrades.md#storage-migrations) is necessary.
+
+This tutorial will use the Substrate Developer Hub
+[Node Template](https://github.com/substrate-developer-hub/substrate-node-template) to explore how to write and perform storage migrations of [FRAME](../../knowledgebase/runtime/frame)-based runtimes.
+> TODO: outline of steps
+
+If you have problems with this tutorial, the Substrate community is full of helpful resources. We
+maintain an active
+[#SubstrateTechnical chat room](https://app.element.io/#/room/!HzySYSaIhtyWrwiwEV:matrix.org) and
+monitor the
+[`substrate` tag on Stack Overflow](https://stackoverflow.com/questions/tagged/substrate). You can
+also use the [`subport` GitHub repository](https://github.com/paritytech/subport/issues/new) to
+create an Issue.
+
+## The Node Template
+
+If you haven't already, you should complete the first three tutorials. The
+[Create Your First Substrate Chain](../../tutorials/create-your-first-substrate-chain/) tutorial
+will guide you through the process of setting up your development environment. The
+[Add a Pallet to Your Runtime](../../tutorials/add-a-pallet) tutorial will introduce the FRAME
+system for runtime development and guide you through the process of extending the capabilities of a
+FRAME runtime by adding a pallet. The [Perform a Forkless Upgrade](../../tutorials/upgrade-a-chain/) tutorial will introduce runtime upgrades.
+
+If you're an experienced developer and wish to skip those
+tutorials, you can clone the
+[Node Template repository](https://github.com/substrate-developer-hub/substrate-node-template) and
+refer to the
+[documentation for local development](https://github.com/substrate-developer-hub/substrate-node-template#local-development).