Detail usage of single and double quoted strings

Add section to contributing guidelines detailing that single quoted
strings should not be used.  The use of it has only resulted in the
addition of more inconsistency to the codebase.

docs/contributing.md

@@ -176,7 +176,19 @@ Code Style
 ----------
 
 The style of the code is a bit of a mixed bag at the moment.  But there
-are at least two things that have been agreed upon.
+are at least a few things that have been agreed upon.
+
+
+### Strings
+
+JavaScript and Lua code should use double quoted strings (`"string"`)
+for all strings with the only exception that strings that contain double
+quotes may use single quotes (i.e., `'String with "quotes"'`.)  For
+JavaScript template literals may also be used where appropriate.
+
+There are places in the codebase that are using `'single'` quoted
+strings.  Should you be modifying such code use double quotes on new and
+modified lines and leave the rest untouched.
 
 
 ### Indenting