chore: Add a basic README to the snap package (#153)

Add a basic README to the snap package

packages/snap/README.md

@@ -0,0 +1,37 @@
+# Metamask Polkadot & Substrate Snap
+
+![](https://github.com/chainsafe/metamask-snap-polkadot/workflows/ci/badge.svg)
+![](https://img.shields.io/github/issues-raw/chainsafe/metamask-snap-polkadot)
+![](https://img.shields.io/github/license/chainsafe/metamask-snap-polkadot)
+
+This project extends Metamask capabilities to include support for Polkadot,
+Kusama, and other Substrate- and FRAME-based blockchains in the Polkadot
+ecosystem.
+
+## What Are Snaps?
+
+> Snaps is a system that allows anyone to safely extend the capabilities of
+> MetaMask. A snap is a program that [Metamask runs] in an isolated environment
+> that can customize the wallet experience.
+>
+> For example, a snap can add new APIs to MetaMask, add support for different
+> blockchain protocols, or modify existing functionality using internal APIs.
+> Snaps is a new way to create web3 end user experiences, by modifying MetaMask
+> in ways that were impossible before.
+
+Learn more about snaps from the
+[official Metamask documentation](https://docs.metamask.io/guide/snaps.html).
+
+**As of 2022-10-01, snaps are only supported in
+[Metamask's developer build, Flask](https://metamask.io/flask/).**
+
+## Documentation
+
+Go to the [wiki](https://github.com/chainsafe/metamask-snap-polkadot/wiki) to
+learn morea about using this snap.
+
+The wiki also describes the
+[integration process](https://github.com/chainsafe/metamask-snap-polkadot/wiki#integration)
+and provides full
+[API documentation](https://github.com/chainsafe/metamask-snap-polkadot/wiki/API-Documentation)
+for dApp developers.