[markdown/en] Added file for learning markdown

The only real problem I encountered in writing this was in the section where I was showing how to do Github styled code embeds and since the majority of my document was inside of a <code>```markdown</code> ... <code>```</code> envelope, trying to embed the example:
<pre>```ruby
def foobar
    puts "Hello World!"
end
```</pre>
wasn't possible as far as I could tell, so I had to escape those backticks. Otherwise it looks pretty good. I mainly used Gruber's post on syntax and Adam P's cheatsheet (both of which were linked at the bottom) as sources.