Add docs for subxt-rpcs and fix example (#1954)

* Add docs for subxt-rpcs and fix example

* Add docs bits in Cargo.toml

* add missing docsrs cfg_attr

---------

Co-authored-by: Niklas Adolfsson <niklasadolfsson1@gmail.com>

rpcs/README.md

@@ -0,0 +1,18 @@
+# subxt-rpcs
+
+This crate provides an interface for interacting with Substrate nodes via the available RPC methods.
+
+```rust
+use subxt_rpcs::{RpcClient, ChainHeadRpcMethods};
+
+// Connect to a local node:
+let client = RpcClient::from_url("ws://127.0.0.1:9944").await?;
+// Use a set of methods, here the V2 "chainHead" ones:
+let methods = ChainHeadRpcMethods::new(client);
+
+// Call some RPC methods (in this case a subscription):
+let mut follow_subscription = methods.chainhead_v1_follow(false).await.unwrap();
+while let Some(follow_event) = follow_subscription.next().await {
+    // do something with events..
+}
+```