Update the intro of tl.tex, and add a reference to VECOS'11.

* doc/tl/tl.tex, doc/tl/tl.bib: Here.

doc/tl/tl.bib

@@ -77,6 +77,26 @@
   note		= {\url{https://es.fbk.eu/people/tonetta/tests/tcad07/}}
 }
 
+@InProceedings{	  duret.11.vecos,
+  author	= {Alexandre Duret-Lutz},
+  title		= {{LTL} Translation Improvements in {Spot}},
+  booktitle	= {Proceedings of the 5th International Workshop on
+		  Verification and Evaluation of Computer and Communication
+		  Systems (VECoS'11)},
+  year		= {2011},
+  series	= {Electronic Workshops in Computing},
+  address	= {Tunis, Tunisia},
+  month		= sep,
+  publisher	= {British Computer Society},
+  abstract	= {Spot is a library of model-checking algorithms. This paper
+		  focuses on the module translating LTL formul{\ae} into
+		  automata. We discuss improvements that have been
+		  implemented in the last four years, we show how Spot's
+		  translation competes on various benchmarks, and we give
+		  some insight into its implementation.},
+  url		= {http://ewic.bcs.org/category/15853}
+}
+
 @Book{		  eisner.06.psl,
   author	= {Cindy Eisner and Dana Fisman},
   title		= {A Practical Introduction to {PSL}},