add readme explanation

README.md

@@ -46,6 +46,8 @@ This tap accepts the following configuration options:
   - `metrics_log_level`
   - `stream_maps`
   - `stream_maps_config`
+  - `milestones`: Parameters to be used by the `milestones` stream:
+    - `state`: Valid options are `open` (default), `closed`, `all`.
   - `rate_limit_buffer` - A buffer to avoid consuming all query points for the auth_token at hand. Defaults to 1000.
   - `expiry_time_buffer` - A buffer used when determining when to refresh Github app tokens. Only relevant when authenticating as a GitHub app. Defaults to 10 minutes. Tokens generated by GitHub apps expire 1 hour after creation, and will be refreshed once fewer than `expiry_time_buffer` minutes remain until the anticipated expiry time.