Add example of emitting tags to documentation.

[davidzchen]

Is there a way to emit explicit tags when emitting YAML values?

The How to Emit YAML documentation does not have any mention of this.

[davidzchen]

After digging through the bugs, I found that tags can be added using YAML::LocalTag("tag").

I'm happy to add this to the documentation.

[jbeder]

Great! Happy to accept a PR.