message: Add doc for sst diff
diff: @@ -135,6 +135,10 @@ In addition to the [global options](#global-options) below, the `deploy` command
  Remove all your stacks and all of their resources from AWS. Or optionally remove a specific stack. Also removes the debug stack that might've been deployed along with `sst start`.
  +### `diff [stacks..]`
  +
  +Compares all the current version of stacks in your app with the already-deployed version. Or optionally compares a list of stacks.
  +
  ### `add-cdk [packages..]`
  Installs the given AWS CDK npm packages with the appropriate CDK version. This convenience method helps get around the [known CDK issue of version mismatches](known-issues.md). This command internally simply does and `npm install` or `yarn add`.
  