README.md in dentaku-0.2.12 vs README.md in dentaku-0.2.13

@@ -46,39 +46,72 @@
     calculator.evaluate('5 + 3 * 2')
     => 11
     calculator.evaluate('(5 + 3) * 2')
     => 16
 
-A number of functions are also supported.  Okay, the number is currently two,
-but more will be added soon.  The current functions are `round` and `if`, and
-they work like their counterparts in Excel:
+A number of functions are also supported.  Okay, the number is currently five,
+but more will be added soon.  The current functions are
+`if`, `not`, `round`, `rounddown`, and `roundup`, and they work like their counterparts in Excel:
 
     calculator.evaluate('if (pears < 10, 10, 20)', :pears => 5)
     => 10
     calculator.evaluate('if (pears < 10, 10, 20)', :pears => 15)
     => 20
 
-`Round` can be called with or without the number of decimal places:
+`round`, `rounddown`, and `roundup` can be called with or without the number of decimal places:
 
     calculator.evaluate('round(8.2)')
     => 8
     calculator.evaluate('round(8.2759, 2)')
     => 8.28
 
+`round` and `rounddown` round down, while `roundup` rounds up.
 
 If you're too lazy to be building calculator objects, there's a shortcut just
 for you:
 
     Dentaku('plums * 1.5', {:plums => 2})
     => 3.0
 
 
-SUPPORTED OPERATORS AND FUNCTIONS
+BUILT-IN OPERATORS AND FUNCTIONS
 ---------------------------------
 
-Math: `+ - * /`  
-Logic: `< > <= >= <> != = AND OR NOT`  
-Functions: `IF ROUND`
+Math: `+ - * / %`
+Logic: `< > <= >= <> != = AND OR`
+Functions: `IF NOT ROUND ROUNDDOWN ROUNDUP`
+
+
+EXTERNAL FUNCTIONS
+------------------
+
+See `spec/external_function_spec.rb` for examples of how to add your own functions.
+
+The short, dense version:
+
+Each rule for an external function consists of three parts: the function's name,
+a list of tokens describing its signature (parameters), and a lambda representing the
+function's body.
+
+The function name should be passed as a symbol (for example, `:func`).
+
+The token list should consist of `:numeric` or `:string` if a single value of the named
+type should be passed; `:non_group` or `:non_group_star` for grouped expressions.
+
+> (what's the difference? when would you use one instead of the other?)
+
+The function body should accept a list of parameters. Each function body will be passed
+a sequence of tokens, in order:
+
+1. The function's name
+2. A token representing the opening parenthesis
+3. Tokens representing the parameter values, separated by tokens representing the commas between parameters
+4. A token representing the closing parenthesis
+
+It should return a token (either `:numeric` or `:string`) representing the return value.
+
+Rules can be set individually using Calculator#add_rule, or en masse using Calculator#add_rules.
+
 
 THANKS
 ------
 
 Big thanks to [ElkStone Basements](http://www.elkstonebasements.com/) for