add README

README.md

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+# etags
+
+Checks urls for etags and reports back any changed urls. Requires [DynamoDb](https://aws.amazon.com/dynamodb/),
+or something like it. For a lightweight, self-hosted version, you can try
+[DynamoDb Bolt](https://github.com/elerch/ddbbolt).
+
+Usage: `etags.py <url> ...`
+
+## Environment variables
+
+  * ETAGS_BUS_NAME: If set, this will notify on an [EventBridge bus](https://aws.amazon.com/eventbridge/)
+  * ETAGS_TABLE: Table name for DynamoDB
+  * DDB_ENDPOINT: By default, the application will use DynamoDb's standard endpoint
+                  Set this variable if using a non-standard endpoint or DynamoDb Bolt
+
+This uses boto3, so all [AWS Environment Variables](https://docs.aws.amazon.com/cli/latest/userguide/cli-configure-envvars.html)
+will control authentication. If using DynamoDb Bolt, the following must be set,
+but can be dummy values:
+
+  * AWS_DEFAULT_REGION
+  * AWS_ACCESS_KEY_ID
+  * AWS_SECRET_ACCESS_KEY
+
+# Running as Docker
+
+Latest version can be found at [https://r.lerch.org/repo/etags/tags/](https://r.lerch.org/repo/etags/tags/).
+Versions are tagged with the short hash of the git commit, and are
+built as a multi-architecture image based on a scratch image.
+
+You can run the docker image with a command like:
+
+```sh
+docker run \
+  --rm                              \
+  --tmpfs /tmp                      \
+  --name=ddbbolt                    \
+  -e AWS_DEFAULT_REGION=us-west-2   \
+  -e AWS_ACCESS_KEY_ID=AKIAEXAMPLE  \
+  -e AWS_SECRET_ACCESS_KEY=dummy    \
+  -e DDB_ENDPOINT=set_if_applicable \
+  -e ETAGS_TABLE=etags              \
+  r.lerch.org/etags:0af6716
+```