docs: Create a README that explains the two clients and the feature

influxdb_iox_client/README.md

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+# InfluxDB IOx Client
+
+This is the official Rust client library for connecting to InfluxDB IOx.
+
+## Using the HTTP API
+
+If you only want to use the HTTP API, create an `influxdb_iox_client::Client`. Here is an example of
+creating an instance that connects to an IOx server running at `http://127.0.0.1:8080` and creating
+a database named `telemetry`:
+
+```rust
+#[tokio::main]
+fn main() {
+    use data_types::database_rules::DatabaseRules;
+    use influxdb_iox_client::ClientBuilder;
+
+    let client = ClientBuilder::default()
+        .build("http://127.0.0.1:8080")
+        .expect("client should be valid");
+
+    client
+        .create_database("telemetry", &DatabaseRules::default())
+        .await
+        .expect("failed to create database");
+}
+```
+
+## Using the Arrow Flight API
+
+IOx supports an [Arrow Flight gRPC API](https://arrow.apache.org/docs/format/Flight.html). To use
+it, enable the `flight` feature, which is off by default:
+
+```toml
+[dependencies]
+influxdb_iox_client = { version = "[..]", features = ["flight"] }
+```
+
+Then you can create an `influxdb_iox_client::FlightClient` by using the `FlightClientBuilder`. Here
+is an example of creating an instance that connects to an IOx server with its gRPC services
+available at `http://localhost:8082` and using it to perform a query:
+
+```rust
+#[tokio::main]
+fn main() {
+    use data_types::database_rules::DatabaseRules;
+    use influxdb_iox_client::FlightClientBuilder;
+
+    let client = FlightClientBuilder::default()
+        .build("http://127.0.0.1:8082")
+        .expect("client should be valid");
+
+    let mut query_results = client
+        .perform_query(scenario.database_name(), sql_query)
+        .await;
+
+    let mut batches = vec![];
+
+    while let Some(data) = query_results.next().await {
+        batches.push(data);
+    }
+}
+```