Add a note to the style guide about abbreviations

resolves #1824

After the section on specific terms, but before "simple language", add a
section that explains that documentation should spell out acronyms,
preferring the "<full-spelling> (<acronym>)" format.

Author: sirosen

documentation/style-guide.rst

@@ -116,6 +116,17 @@ Unix
    1970s.
 
 
+Spell out acronyms
+==================
+
+The first time an acronym is used on a page, spell it out.
+Prefer to write out the full term and follow it with the acronym in parentheses.
+For example, write "JavaScript Object Notation (JSON)".
+
+Avoid using the ``:abbr:`` role, as it generates HTML which is not accessible to
+some forms of assistive technology and mobile users.
+
+
 Use simple language
 ===================