qt strings: spell out how to escape { in qt string

Author: rjbs

rfcs/rfc0019.md

@@ -54,6 +54,8 @@ expression to be interpolated into the string in place of the `{...}`
 construct.  This is arbitrarily extensible for other existing expressions and
 for new ones that may be added.
 
+To include a verbatim `{` in the string value, escape it:  `\{`
+
 ## Specification
 
 This: