Explain how to add a sample video locally

I'm sure there are lots of way to say things better, please do so if you want. I can also improve this if given more instructions.
This block with the triple backticks needs to be on a new line. Otherwise it becomes a massively long single line that wraps strangely. 

Also, you can remove every inside the `<iframe>` tag. I.e. all that `<a target=...`.