Add a section explaining comments
Change commented out form to use `comment` macro

By changing from semi-colon comments to the `comment` macro, users can
still attempt to evaluate the form to prove that it won't compile.
Fix a typo
add optional, non posizional and defaulted args in the destructoring section.
mention lein
Merge pull request #13 from magomimmo/patch-02

add optional, non positional and defaulted args in map destructuring
Merge pull request #12 from jballanc/comments

Add a section explaining comments