DOCSP-11513: [CHARTS] Can't create a calculated filed whose value is a literal string (#395)

Author: steveren

source/calculated-fields.txt

@@ -182,6 +182,20 @@ complex calculated fields.
 
       { $sum: ["$price", "$salesTax"] }
 
+Literal Strings
+~~~~~~~~~~~~~~~
+
+You can set a calculated field's value to a string using the ``$literal``
+operator.
+
+.. example::
+   
+   The following field definition assigns a value of ``my string``:
+
+   .. code-block:: json
+
+      { $literal: "my string" }
+
 .. _edit-calculated-field:
 
 Edit a Calculated Field