Update go-client startworkflow options with First Run At Parameter (#244)

Author: timl3136

docs/05-go-client/03-starting-workflows.md

@@ -181,5 +181,10 @@ type StartWorkflowOptions struct {
 	// This works with CronSchedule and with DelayStart.
 	// Optional: defaulted to 0 seconds
 	JitterStart time.Duration
+
+	// FirstRunAt - Specific time (in RFC 3339 format) to let the first run of the workflow to start at,
+	// This will only be used and override DelayStart and JitterStart if provided in the first run
+	// Optional: defaulted to Unix epoch time
+	FirstRunAt time.Time
 }
-```
+```