Update README.md (#680)

* Update README.md

Added introductory text on the api-client

* Update intruction text

README.md

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 # substrate-api-client
 
-substrate-api-client is a library written in Rust for connecting to the substrate's RPC interface via WebSockets allowing to
+The substrate-api-client is a Rust library for connecting to a [substrate](https://substrate.io/)-based node via RPC. It's particularly useful for setups with no-std environment (which are typical for trusted execution environmnets or embedded devices). It provides similar functionalities as [Polkadot-js](https://wiki.polkadot.network/docs/polkadotjs), such as easy extrinsic submission and state queries. With an RPC client, developers can easily interact with any [Polkadot](https://polkadot.network/) or [Kusama](https://kusama.network/) chain. There are several [RPC clients](https://wiki.polkadot.network/docs/build-tools-index#rpc-and-api-tools) available in different programming languages. For Rust, the most popular RPC client is [subxt](https://github.com/paritytech/subxt). The substrate-api-client provides a simpler, less extensive alternative to subxt, focused on providing as many features as possible for no-std environments.
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+The substrate-api-client connects to the substrate's RPC interface via WebSockets allowing to
 
 * Compose extrinsics, send them (asynchronously and synchronously) and subscribe to updates (synchronously).
 * Support `no_std` builds. Only the rpc-client is std only. For `no_std` builds, a custom rpc client needs to be implemented.