docs: Add Cloudflare R2 example (#504)

* docs: Add Cloudflare R2 example

* edit

Author: kylebarron

docs/assets/cloudflare-r2-bucket-info.png

@@ -0,0 +1,48 @@
+# Cloudflare R2
+
+[Cloudflare R2](https://www.cloudflare.com/developer-platform/products/r2/) is Cloudflare's object storage solution, designed to be compatible with the S3 API. Some developers may choose to use Cloudflare R2 because it has no egress fees.
+
+It's easy to read and write data to and from Cloudflare R2 with obstore's [`S3Store`][obstore.store.S3Store] with three steps:
+
+1. [Create an API token](https://dash.cloudflare.com/?to=/:account/r2/api-tokens) with read or read/write access to one or more buckets.
+
+    Copy the `Access Key ID` and `Secret Access Key` to use in your code.
+
+    ![](../assets/cloudflare-r2-credentials.jpg)
+
+2. On the general settings of a bucket, take note of the `S3 API` URL. In my case it's `https://f0b62eebfbdde1133378bfe3958325f6.r2.cloudflarestorage.com/kylebarron-public`.
+
+    ![](../assets/cloudflare-r2-bucket-info.png)
+
+3. Pass this information to [`S3Store.from_url`][obstore.store.S3Store.from_url]:
+
+    ```py
+    from obstore.store import S3Store
+
+    access_key_id = "..."
+    secret_access_key = "..."
+    store = S3Store.from_url(
+        "https://f0b62eebfbdde1133378bfe3958325f6.r2.cloudflarestorage.com/ kylebarron-public",
+        access_key_id=access_key_id,
+        secret_access_key=secret_access_key,
+    )
+    store.list_with_delimiter()
+    ```
+
+    Or you can construct a store manually with the `endpoint` and `bucket` parameters:
+
+    ```py
+    from obstore.store import S3Store
+
+    access_key_id = "..."
+    secret_access_key = "..."
+    bucket = "kylebarron-public"
+    endpoint = "https://f0b62eebfbdde1133378bfe3958325f6.r2.cloudflarestorage. com"
+    store = S3Store(
+        bucket,
+        access_key_id=access_key_id,
+        secret_access_key=secret_access_key,
+        endpoint=endpoint,
+    )
+    store.list_with_delimiter()
+    ```

docs/assets/cloudflare-r2-credentials.jpg

@@ -40,6 +40,7 @@ nav:
       - examples/fastapi.md
       - examples/minio.md
       - examples/pyarrow.md
+      - examples/r2.md
       - examples/stream-zip.md
       - examples/tqdm.md
       - examples/zarr.md