Include tip on how to add single and multi-word tags to posts

Clearly detail how to properly separate multiple tags on a post
by the use of commas. Follows convention mentioned in help section
of SEO::Titles.

If a user needs to add multi-word tag, then it should also tell
them how to do so by use of double quotes. Otherwise they may
not expect to see it broken up into individual words as spaces
also can separate tags.
Merge pull request #144 from randomecho/article-tagging

Include tip on how to add single and multi-word tags to posts