Document Serai's Application Calls and update Instructions accordingly

docs/Serai.md

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+# Serai
+
+Serai is a decentralization execution layer whose validators form multisig
+wallets for various connected networks, offering secure decentralized custody of
+foreign assets to applications built on it.
+
+Serai is exemplified by Serai DEX, an automated-market-maker (AMM) decentralized
+exchange, allowing swapping BTC, ETH, USDC, DAI, and XMR. It is the premier
+application of Serai.
+
+### Substrate
+
+Serai is based on [Substrate](https://docs.substrate.io), a blockchain framework
+offering a robust infrastructure.
+
+### Smart Contracts
+
+Serai offers WASM-based smart contracts. All applications are built over these
+contracts, enabling composable interactions within a mutual layer. These
+contracts are primarily written in [ink!](https://ink.substrate.io/), a
+framework for building contracts in Rust.
+
+Initially, smart contract deployment will not be enabled. Solely Serai DEX will
+be available, due to the variety of economic considerations around securing the
+multisig. Serai may expand in the future with more explicitly added
+applications, each with tailored economic models, or may enable arbitrary
+contract deployment. At this time, we solely plan for Serai DEX's availabiliy.
+
+### Application Calls
+
+Applications, such as Serai DEX, may be called via calling their relevant smart
+contracts. At a low level, this is done via specifying the address of the
+contract being interacted with, along with SCALE-encoded calldata.