Add !expr to readme and concat/concat* functions

README.md

@@ -237,9 +237,45 @@ Resources:
 
 The true/false values can include maps/arrays/any other valid YAML.
 
+#### !expr
+
+Evaluate an expression and insert the result.
+
+Expects a scalar representing an expression using the same notatation as !if (and
+outlined below).
+
+If the expression evaluates down to a single value, it will be inserted as a
+scalar. If it evaluates down to multiple values, it will be inserted as an array.
+
+Example:
+
+```
+cfnpp:
+  variables:
+    A: 1
+    B: 2
+    C: !expr 1 2 gt
+
+Math:
+  IsALessThanB: !expr A B lt
+  AllMyFavouriteVariables: !expr A B
+  ValueOfC: !var C
+```
+
+Result:
+
+```
+Math:
+  IsALessThanB: True
+  AllMyFavouriteVariables:
+    - 1
+    - 2
+  ValueOfC: False
+```
+
 #### Expressions
 
-Expressions are composed of space separated values and operators. Value components
+Expressions are composed of space separated values and operators. Valid components
 are:
 
 * Numeric Literals: Any integer or float value.
@@ -256,6 +292,9 @@ are:
 	- lte: less than or equal
 	- and: boolean and
 	- or: boolean or
+* Function: Currently supported functions are:
+	- concat: Concatenate as strings two values off of the stack
+	- concat\*: Concatenate all values currently on the stack down to a single value.
 
 ## Usage