Bulk out the README a bit

Give a better idea of what this can for people who haven't clicked through to the full documentation.

README.md

@@ -10,4 +10,31 @@ A tool for creating credentials for accessing S3 buckets
 
 For project background, see [s3-credentials: a tool for creating credentials for S3 buckets](https://simonwillison.net/2021/Nov/3/s3-credentials/) on my blog.
 
-Full documentation: https://s3-credentials.readthedocs.io/
+## Installation
+
+    pip install s3-credentials
+
+## Basic usage
+
+To create a new S3 bucket and output credentials that can be used with only that bucket:
+```
+% s3-credentials create my-new-s3-bucket --create-bucket
+Created bucket:  my-new-s3-bucket
+Created user: s3.read-write.my-new-s3-bucket with permissions boundary: arn:aws:iam::aws:policy/AmazonS3FullAccess
+Attached policy s3.read-write.my-new-s3-bucket to user s3.read-write.my-new-s3-bucket
+Created access key for user: s3.read-write.my-new-s3-bucket
+{
+    "UserName": "s3.read-write.my-new-s3-bucket",
+    "AccessKeyId": "AKIAWXFXAIOZOYLZAEW5",
+    "Status": "Active",
+    "SecretAccessKey": "...",
+    "CreateDate": "2021-11-03 01:38:24+00:00"
+}
+```
+The tool can do a lot more than this. See the [documentation](https://s3-credentials.readthedocs.io/) for details.
+
+## Documentation
+
+- [Full documentation](https://s3-credentials.readthedocs.io/)
+- [Command help reference](https://s3-credentials.readthedocs.io/en/stable/help.html)
+- [Release notes](https://github.com/simonw/s3-credentials/releases)