added note about testing to doc/index feature list

doc/index.txt

@@ -11,6 +11,7 @@ arrays efficiently. Theano features:
 * **symbolic differentiation** -- Let Theano do your derivatives.
 * **speed and stability optimizations** -- Get the right answer for ``log(1+x)`` even when ``x`` is really tiny.
 * **dynamic C code generation** -- Evaluate expressions faster.
+* **extensive testing and self-verification** -- We take correctness seriously.
 
 Theano has been powering large-scale computationally intensive scientific investigations
 since 2007.  But it is also approachable enough to be used in the classroom