Markdown extension docs should include instructions to "include" the components

[goblinfactory]

If a markdown extension is created, but not included, you will get a runtime error (no build error) and it will be invisibly output to the chrome log;

Steps to reproduce

1. create a simple component in new folder /components/foo.html

<div @name="foo">
<h1>hello</h1>
</div

2. Edit any markdown file in a simple-blog starter project, and add [Foo] anywhere on the page.
3. Browse to that page. Press F12, observe error in console.

I believe this is quite important because any new user following the getting started docs is quite likely to end up here at some point during playing with Nue for the first time.

More details in related issue #351

Alternative paths

1. Simply mispelling an existing extension that has correctly been included, will also result in the same obtuse error; for example [MyAccordian] instead of [MyAccordion], which would be incredibly difficult for a human to spot the error with the focus of debugging efforts looking at MyAccordion code which could be totally fine.