Document new option

text_formatter.go

diff --git a/text_formatter.go b/text_formatter.go
index 6df95ea..ac0b01e 100644
--- a/text_formatter.go
+++ b/text_formatter.go
@@ -32,8 +32,10 @@
 
 type TextFormatter struct {
 	// Set to true to bypass checking for a TTY before outputting colors.
-	ForceColors      bool
-	DisableColors    bool
+	ForceColors   bool
+	DisableColors bool
+	// Set to true to disable timestamp logging (useful when the output
+	// is redirected to a logging system already adding a timestamp)
 	DisableTimestamp bool
 }