docs: Add pointer to avocado-framework-tests in WritingTests

In WritingTests documentation we mention `examples/tests`, but users
will probably get a better source of information in
`avocado-framework-tests` repository so let's include both here.
Signed-off-by: NLukáš Doktor <ldoktor@redhat.com>

docs/source/WritingTests.rst

@@ -1707,6 +1707,8 @@ We recommend you take a look at the example tests present in the
 ``examples/tests`` directory, that contains a few samples to take some
 inspiration from. That directory, besides containing examples, is also used by
 the Avocado self test suite to do functional testing of Avocado itself.
+Although one can inspire in `<https://github.com/avocado-framework-tests>`__
+where people are allowed to share their basic system tests.
 
 It is also recommended that you take a look at the :ref:`api-reference`.
 for more possibilities.