add note about secrets manager (#504)

#267 added secret manager support, so I updated the readme to note that. That way people won't need to dig through the issues to figure out what the support status is.

README.md

@@ -440,6 +440,14 @@ you can use `CHAMBER_AWS_REGION` to override just for chamber.
 If you'd like to use a custom SSM endpoint for chamber, you can use `CHAMBER_AWS_SSM_ENDPOINT`
 to override AWS default URL.
 
+## AWS Secrets Manager
+Chamber supports AWS Secrets Manager as an optional backend. For example:
+
+```
+chamber -b secretsmanager write myservice foo fah
+chamber -b secretsmanager write myservice foo2 fah2
+```
+
 ## S3 Backend (Experimental)
 
 By default, chamber store secrets in AWS Parameter Store. We now also provide an
@@ -512,4 +520,4 @@ a github release.
 
 <div align="center">
 THE CHAMBER OF SECRETS HAS BEEN OPENED
-</div>
+</div>