Add better description to list-item-spacing

Closes GH-78.

Author: wooorm

doc/rules.md

@@ -1112,6 +1112,11 @@ When `'invalid'` is passed in, the following error is given:
 Warn when list looseness is incorrect, such as being tight
 when it should be loose, and vice versa.
 
+According the [markdown-style-guide](http://www.cirosantilli.com/markdown-style-guide/),
+if one or more list-items in a list spans more than one line,
+the list is required to have blank lines between each item.
+And otherwise, there should not be blank lines between items.
+
 When this rule is turned on, the following file
 `valid.md` is ok:
 

lib/rules/list-item-spacing.js

@@ -7,6 +7,11 @@
  *   Warn when list looseness is incorrect, such as being tight
  *   when it should be loose, and vice versa.
  *
+ *   According the [markdown-style-guide](http://www.cirosantilli.com/markdown-style-guide/),
+ *   if one or more list-items in a list spans more than one line,
+ *   the list is required to have blank lines between each item.
+ *   And otherwise, there should not be blank lines between items.
+ *
  * @example {"name": "valid.md"}
  *
  *   A tight list: