Update user-guide.rst

Author: Evolutionary-Intelligence

docs/sphinx/source/user-guide.rst

@@ -1,6 +1,14 @@
 User Guide
 ==========
 
+.. note:: "`In our opinion, the main fact, which should be known to any person dealing with
+   optimization models, is that in general, optimization problems are unsolvable. This
+   statement, which is usually missing in standard optimization courses, is very important
+   for understanding optimization theory and the logic of its development in the past and
+   in the future. <https://link.springer.com/book/10.1007/978-3-319-91578-4>`_"---From
+   `Prof. Yurii Nesterov (International Member of National Academy of Sciences, USA
+   <https://www.nasonline.org/directory-entry/yurii-e-nesterov-5n5mo7/>`_
+
 Before applying this open-source library `PyPop7` in `PyPI <https://pypi.org/project/pypop7/>`_
 to real-world black-box optimization (BBO) problems, four basic information should be read
 sequentially, as presented in the following: