docs(snap): add little page explaining how to build

docs/docs/developers/metamask-snap.md

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+# MetaMask Snap
+
+Warden Protocol maintains a [MetaMask Snap](https://metamask.io/snaps/) to
+bring your Warden Protocol Ethereum addresses to any dApp that already supports
+MetaMask.
+
+![MetaMask accounts list with a Warden Protocol controlled address](./metamask-snap-accounts-preview.jpg)
+
+
+## Build
+
+Clone the Warden Protocol repo:
+
+```sh
+git clone https://github.com/warden-protocol/wardenprotocol
+
+cd wardenprotocol/snap
+```
+
+Install the required dependencies:
+
+```sh
+npm install
+```
+
+Run the server (it will monitor changes and rebuild automatically):
+
+```sh
+npm start
+```
+
+Now SpaceWard can load the MetaMask Snap using the origin
+`local:http://localhost:8123`.
+
+
+## Publish
+
+MetaMask Snaps are published as npm packages. At this time, the Warden Protocol
+MetaMask Snap is not published to npm, but you can build and test it
+locally.