Explain that lists and bullets require empty line before

When using markdown in the real world, one common and frustrating issue when using bullets and lists is that a blank line is not left before the list - this causes them to fail to render correctly.

For lesson 6, it would be valuable to explain this as well as have an exercise to test this much needed markdown knowledge.

Example which doesn't render

Markdown:

Here are your bullets, sir:
- Lead
- Jacketed lead
- Hard Cast
- Blank

Rendered markdown:

Example which renders

Markdown:

Here are your bullets, sir:

- Lead
- Jacketed lead
- Hard Cast
- Blank

Rendered Markdown:

Open lesson 6 at http://markdowntutorial.com/lesson/6/ and review how its Markdown explanation and exercise are structured. Add the missing guidance that lists and bullets need a preceding blank line, along with an exercise covering both examples. Done means lesson 6 clearly explains the rendering difference and tests this knowledge.