Update the README for @solana/rpc-api (#2994)

Addresses #2983.

packages/rpc-api/README.md

@@ -14,4 +14,46 @@
 
 # @solana/rpc-api
 
-TODO
+This package contains types that describe the [methods](https://solana.com/docs/rpc/http) of the Solana JSON RPC API, and utilities for creating a `RpcApi` implementation with sensible defaults. It can be used standalone, but it is also exported as part of the Solana JavaScript SDK [`@solana/web3.js@experimental`](https://github.com/solana-labs/solana-web3.js/tree/master/packages/library).
+
+Each RPC method is described in terms of a TypeScript interface of the following form:
+
+```ts
+interface ExampleApi extends RpcApiMethods {
+    getSomething(address: Address): Something;
+}
+```
+
+A `RpcApi` that implements `ExampleApi` will ultimately expose its defined methods on any `Rpc` that uses it.
+
+```ts
+const rpc: Rpc<ExampleApi> = createExampleRpc(/* ... */);
+const something: Something = await rpc.getSomething(address('95DpK3y3GF7U8s1k4EvZ7xqyeCkhsHeZaE97iZpHUGMN')).send();
+```
+
+## Types
+
+### `SolanaRpcApi{Devnet|Testnet|Mainnet}`
+
+These types represent the RPC methods available on a specific Solana cluster.
+
+For instance, the test clusters support the `RequestAirdropApi` while mainnet does not.
+
+## Functions
+
+### `createSolanaRpcApi(config)`
+
+Creates a `RpcApi` implementation of the Solana JSON RPC API with some default behaviours.
+
+The default behaviours include:
+
+-   A transform that converts `bigint` inputs to `number` for compatiblity with version 1.0 of the Solana JSON RPC.
+-   A transform that calls the config's `onIntegerOverflow` handler whenever a `bigint` input would overflow a JavaScript IEEE 754 number. See [this](https://github.com/solana-labs/solana-web3.js/issues/1116) GitHub issue for more information.
+-   A transform that applies a default commitment wherever not specified
+
+#### Arguments
+
+A config object with the following properties:
+
+-   `defaultCommitment`: An optional default `Commitment` value. Given an RPC method that takes `commitment` as a parameter, this value will be used when the caller does not supply one.
+-   `onIntegerOverflow(methodName, keyPath, value): void`: An optional function that will be called whenever a `bigint` input exceeds that which can be expressed using JavaScript numbers. This is used in the default `SolanaRpcApi` to throw an exception rather than to allow truncated values to propagate through a program.