Merge pull request puppetlabs#532 from DavidS/fix-is_a-readme

(FM-3701) Update README for is_a

Author: Helen

README.markdown

@@ -405,7 +405,7 @@ Returns an array an intersection of two. For example, `intersection(["a","b","c"
 
 #### `is_a`
 
-Boolean check to determine whether a variable is of a given data type. This is equivalent to the `=~` type checks.
+Boolean check to determine whether a variable is of a given data type. This is equivalent to the `=~` type checks. This function is only available in Puppet 4, or when using the "future" parser. 
 
   ~~~
   foo = 3