Document rate limit. (#52)

README.md

@@ -22,6 +22,7 @@ Pantheon’s Secrets Manager Terminus plugin is key to maintaining industry best
   * [Site secrets Commands](#site-secrets-commands)
   * [Organization secrets Commands](#organization-secrets-commands)
   * [Help](#help)
+- [Rate Limiting](#rate-limiting)
 - [Use Secrets with Integrated Composer](#use-secrets-with-integrated-composer)
   * [Mechanism 1: Oauth Composer authentication](#mechanism-1-oauth-composer-authentication)
   * [Mechanism 2: HTTP Basic Authentication](#mechanism-2-http-basic-authentication)
@@ -429,6 +430,10 @@ terminus secret:org:delete --env=<env> <org> <secret-name>
 
 Run `terminus list secret` for a complete list of available commands. Use terminus help <command> to get help with a specific command.
 
+## Rate Limiting
+
+The service supports up to 3 requests per second per user. If you hit that limit, the API will return a `429` error code and the plugin will throw an error.
+
 ## Use Secrets with Integrated Composer
 
 You must configure your private repository and provide an authentication token before you can use the Secrets Manager Terminus plugin with Integrated Composer. You could use either of the following mechanisms to setup this authentication.