Add comment to log function

Signed-off-by: Marcos Lilljedahl <marcosnils@gmail.com>

entry.go

diff --git a/entry.go b/entry.go
index 001c4c7..04673a0 100644
--- a/entry.go
+++ b/entry.go
@@ -70,6 +70,8 @@
 	return &Entry{Logger: entry.Logger, Data: data}
 }
 
+// This function is not declared with a pointer value because otherwise
+// race conditions will occur when using multiple goroutines
 func (entry Entry) log(level Level, msg string) {
 	entry.Time = time.Now()
 	entry.Level = level