pkg/watcher: update REAME descriptions
pkg/watcher: update REAME descriptions

TBR

Review URL: https://codereview.chromium.org//439713007

git-svn-id: c53bd6455474c0a826d309663cf600546f7b0963@38863 260f80e4-7a28-3924-810f-c04153c831b5

dart-lang/watcher