Reference links
An explicit reference link has two parts, the link itself and the link definition, which may occur elsewhere in the document (either before or after the link).
The link consists of link text in square brackets, followed by a label in square brackets. (There can be space between the two.) The link definition must begin at the left margin or indented no more than three spaces. It consists of the bracketed label, followed by a colon and a space, followed by the URL, and optionally (after a space) a link title either in quotes or in parentheses.
Here are some examples:
[my label 1]: /foo/bar.html "My title, optional"
[my label 2]: /foo
[my label 3]: http://fsf.org (The free software foundation)
[my label 4]: /bar#special 'A title in single quotes'
The URL may optionally be surrounded by angle brackets:
[my label 5]: <http://foo.bar.baz>
The title may go on the next line:
[my label 3]: http://fsf.org
"The free software foundation"
Note that link labels are not case sensitive. So, this will work:
Here is [my link][FOO]
[Foo]: /bar/baz
In an implicit reference link, the second pair of brackets is empty, or omitted entirely:
See [my website][], or [my website].
[my website]: http://foo.bar.baz
