update readme

README.md

@@ -1,2 +1,28 @@
 # aws-target-group-cleanup
 Remove ALB target groups that are not assigned to load balancers
+
+# Purpose
+Clean up target groups that no longer have a load balancer assigned
+
+
+# Prerequisites
+* `pip install boto3`
+* Either an AWS role (if running on EC2) or an access key/secret key
+
+# Usage
+
+Dry run mode:
+```bash
+python aws-target-group-cleanup.py -r us-west-2 -p myprefix
+```
+
+Removal mode:
+```bash
+python aws-target-group-cleanup.py -r us-west-2 -p myprefix -f
+```
+
+Where:
+
+* -r is the region
+* -p is an optional prefix (ie. only remove target groups starting with this)
+* -f will turn on the removal (run in dryrun mode without this flag)