README.md: improve $variable interpolation section

README.md

@@ -3,8 +3,8 @@ PBot
 PBot is a versatile IRC Bot written in Perl
 
 <!-- md-toc-begin -->
-  * [Installation / Quick Start](#installation--quick-start)
-  * [Features](#features)
+* [Installation / Quick Start](#installation--quick-start)
+* [Features](#features)
   * [Commands](#commands)
   * [Plugins](#plugins)
   * [Factoids](#factoids)
@@ -19,11 +19,11 @@ PBot is a versatile IRC Bot written in Perl
     * [piping](#piping)
     * [command substitution](#command-substitution)
     * [command splitting](#command-splitting)
-      * [$variable interpolation](#variable-interpolation)
+    * [advanced $variable interpolation](#advanced-variable-interpolation)
     * [inline commands](#inline-commands)
-  * [Documentation](#documentation)
-  * [Support](#support)
-  * [License](#license)
+* [Documentation](#documentation)
+* [Support](#support)
+* [License](#license)
 <!-- md-toc-end -->
 
 Installation / Quick Start
@@ -291,7 +291,7 @@ You can execute multiple commands sequentially as one command.
     <pragma-> !echo Test! ;;; me smiles. ;;; version
        <PBot> Test! * PBot smiles. PBot version 2696 2020-01-04
 
-#### $variable interpolation
+#### advanced $variable interpolation
 
 You can use factoids as variables and interpolate them within commands.
 
@@ -300,6 +300,12 @@ You can use factoids as variables and interpolate them within commands.
     <pragma-> !echo greeting is $greeting
        <PBot> greeting is Hello, world
 
+PBot `$variable` interpolation supports [expansion modifiers](doc/Factoids#expansion-modifiers), which can be chained to
+combine their effects.
+
+    <pragma-> !echo $greeting:uc
+       <PBot> HELLO, WORLD
+
 #### inline commands
 
 You can invoke up to three commands inlined within a message.  If the message