Add interact.md (#2)

Author: liuchengxu

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+# Interact with Subcoin Chain
+
+The [Polkadot-JS Apps](https://polkadot.js.org/apps) is a versatile UI for engaging with a Substrate based node. You can connect to it to inspect the state of UTXO set, call RPCs, potentially transfer coins in the future.
+
+To connect the Polkadot-JS Apps to your local node, open the sidebar and switch to `Development` > `Local Node (127.0.0.1:9944)`.
+
+![Connect to Local Node](./img/connect-to-local-node.png)
+
+## Inspect the State of UTXO Set
+
+Assuming the current best block is #125,954, you may want to examine the UTXO of the coinbase transaction for this block. The Bitcoin transaction ID for this coinbase transaction is `4ae4acf6fcdfec212de5fdcb7e58028764032aacaf194754b7e60fd842acd92f`. You can view it [here](https://www.blockchain.com/explorer/transactions/btc/4ae4acf6fcdfec212de5fdcb7e58028764032aacaf194754b7e60fd842acd92f).
+
+To look up the UTXO:
+
+1. Go to `Developers` > `Chain State`.
+2. Paste the txid into the `H256` text box, note that `0x` prefix is necessary.
+3. Click the `+` button on the right side to view the details of the coin.
+
+![Coin Storage](./img/coin-storage.png)
+
+If the node you are connected to is an archive node, you will be able to inspect any UTXO at any height throughout the history.
+
+## Additional Resources
+
+- https://polkadot.js.org/docs/substrate/
+- https://docs.substrate.io/reference/command-line-tools/polkadot-apps/