added documentation for the third supported boolean operator (NOT)

added documentation for the third boolean operator (NOT) which is supported by the 'custom export filter'

additionally, provided an example for the usage of the NOT operator and the corresponding text output

en/CustomExports.md

@@ -75,15 +75,20 @@ However, if the `editor` field has not been set - it might not even make sense f
 
 The `\begin` and `\end` commands make sure the text in between is printed if and only if the field referred in the curly braces is defined for the entry being exported.
 
-A conditional block can also be dependent on more than one field, and the content is only printed when simple boolean conditions are satisfied. Two boolean operator are provided:
+A conditional block can also be dependent on more than one field, and the content is only printed when simple boolean conditions are satisfied. Three boolean operators are provided:
 
 -   AND operator : `&`, `&&`
 -   OR operator : `|`, `||`
+-   NOT operator : `!`
 
-To output text only if both `year` and `month` are set, use a block like the following:
-`\begin{year&&month}Month: \format[HTMLChars]{\month}\end{year&&month}`
+For example, to output text only if both `year` and `month` are set, use a block like the following:
+`\begin{year&&month}Month: \format[HTMLChars]{\month}\end{year&&month}`  
 which will print "Month: " plus the contents of the `month` field, but only if also the `year` field is defined.
 
+As an example for the usage of the NOT operator, consider the following:  
+`\begin{!year}\format[HTMLChars]{(no year)}\end{!year}`  
+Here, "no year" is printed as output text if no year field is defined. 
+
 **Note:** Use of the `\begin` and `\end` commands is a key to creating layout files that work well with a variety of entry types.
 
 ### Grouped output