How to link to a category
To add a link to a category in an article (as opposed to adding the article to a category) create a normal link to the category page but place a colon at the beginning of it, just after the double opening square brackets. Here's an example:
To provide a link to Category:Animals (just like this one), add the text [[:Category:Animals]] where you want the link to appear. (Notice the extra colon at the beginning of the link).