Update README.md

Added str_swapcase to list of functions.

Author: eengl

README.md

@@ -20,6 +20,7 @@ The function names are prefixed with ``str_``.  The following is a list of avail
 * **str_center** - Center a string to a specified width.  The default character to fill in the centered string is a blank character.
 * **str_reverse** - Reverse a string.
 * **str_test** - Return .true. is a substring is found in a string, .false. otherwise.
+* **str_swapcase** - Swap the case of letters in a string.
 
 All functions return a deferred-length, allocatable character scalar (``character(len=:), allocatable``) with the exception of **``str_count``** which returns an integer and **``str_test``** which returns a logical.