Add xss notes/examples to templates section in documentation

[wickkidd]

Adding the notes and examples from #1010 to the template() option function documentation (https://github.com/visjs/vis-timeline/blob/master/docs/timeline/index.html#LL2071C15) would help prevent a lot of confusion as to why it doesn't work according to the current documentation.

[Jimm31]

I lost about two hours looking for why I cut HTML tags in the template. Please add a note to the documentation that the xss: {disabled: true} parameter works only if you pass it in the constructor!!!