update README.md add *.tmpl file example (#203)

cortextool alertmanager load template files

Author: justinTM

README.md

@@ -55,6 +55,8 @@ The following commands are used by users to interact with their Cortex alertmana
 
     cortextool alertmanager load ./example_alertmanager_config.yaml
 
+    cortextool alertmanager load ./example_alertmanager_config.yaml template_file1.tmpl template_file2.tmpl
+
 #### Rules
 
 The following commands are used by users to interact with their Cortex ruler configuration. They can load prometheus rule files, as well as interact with individual rule groups.