Add VSTR reference.

README.md

@@ -176,7 +176,7 @@ For the macro-preprocessing part, other libraries also exist. For example:
 For the string library, there is for example:
 
 * [The Better String Library](http://bstring.sourceforge.net/) (with a page which lists a lot of other string libraries).
-
+* [VSTR](http://www.and.org/vstr/) with a [page](http://www.and.org/vstr/comparison) which lists a lot of other string libraries.
 
 How to use
 ----------