Add autodetect event flag (#486)

* Add autodetect event flag

* Add new flag to README.md

* Make help more clear

README.md

@@ -52,7 +52,8 @@ or in a shell by running
 
 ```
 # Command structure:
-act [event name to run] [flags]
+act [<event>] [options]
+If no event name passed, will default to "on: push"
 
 # List the actions for the default event:
 act -l
@@ -85,6 +86,7 @@ act -v
   -C, --directory string       working directory (default ".")
   -n, --dryrun                 dryrun mode
       --env-file string        environment file to read and use as env in the containers (default ".env")
+      --detect-event           Use first event type from workflow as event that triggered the workflow
   -e, --eventpath string       path to event JSON file
   -g, --graph                  draw workflows
   -h, --help                   help for act