Document "-" key.

goyaml.go

diff --git a/goyaml.go b/goyaml.go
index eb2a29f..1b71c8e 100644
--- a/goyaml.go
+++ b/goyaml.go
@@ -119,6 +119,8 @@
 //     flow         Marshal using a flow style (useful for structs,
 //                  sequences and maps.
 //
+// In addition, if the key is "-", the field is ignored.
+//
 // For example:
 //
 //     type T struct {