t.select manual: add scientific paper ref

temporal/t.select/t.select.html

@@ -367,6 +367,9 @@ D = if(contains, td(buff_t(A, "1 days")) == 3, B, C)
 <h2>REFERENCES</h2>
 
 <a href="http://www.dabeaz.com/ply/">PLY(Python-Lex-Yacc)</a>
+<p>
+Gebbert, S., Leppelt, T., Pebesma, E., 2019. A topology based spatio-temporal map algebra for
+big data analysis. Data 4, 86. <a href="https://doi.org/10.3390/data4020086">https://doi.org/10.3390/data4020086</a>
 
 <h2>AUTHORS</h2>