Explain the usage of av skip tag.

Related to trivial edits on documentation source files #11766 (comment)

Improved explanation.

Fix typo

[av skip]

CONTRIBUTING.md

@@ -72,7 +72,8 @@ Julia's documentation is stored in the `doc` directory, and like everything else
 - Search for the function you want to change
 - Make your changes
 - Exit Zen mode
-- Provide a title, and optionally a longer description of your change
+- Provide a title
+- Provide a longer description of your change with the tag `[av skip]` included at the bottom (this is used in order to prevent the AppVeyor job queue from growing too large because of documentation edits)
 - Submit your change
 
 Julia's documentation is built with [Sphinx](http:https://sphinx-doc.org/contents.html), which supports (and Julia's docs rely heavily on) [ReST directives](http:https://docutils.sourceforge.net/docs/ref/rst/directives.html). To build the documentation locally, run