Fix spelling and phrasing.

Simple transpositions in spots. Updated last sentence to something I
consider more concise and clear.
Correct a semicolon to a colon.

Man, that's pushing it isn't it!
Add a section, correct problem statement.

The problem was introduced as testing the membership of
multiple items as well as single items. But multiple items was never
discussed, and the recipe was written extremely well for single
items. Multiple items should have their own recipe, since this one also explores using sets as functions.
Merge https://github.com/clojure-cookbook/clojure-cookbook
Add license agreement.
Merge pull request #19 from JouisWalker/master

Typo fixes to composite-data/vectors/adding-an-item.asciidoc and subsection contribution to sets