Include docs/requirements.txt in source distributions

MANIFEST.in

@@ -6,7 +6,7 @@ include README.rst
 recursive-include cyipopt *.py *.pyx *.pxd
 recursive-include ipopt *.py
 recursive-include tests *.py
-recursive-include docs Makefile *.bat *.rst *.py
+recursive-include docs Makefile *.bat *.rst *.txt *.py
 recursive-include examples *.py
 exclude cyipopt/tests/unit/test_scipy_ipopt_from_scipy.py
 prune include*