Add links to the three Verilog standards

Author: kroening

src/verilog/README

@@ -1,3 +1,12 @@
+Verilog 1995:
+https://ieeexplore.ieee.org/document/803556
+
+Verilog 2001:
+https://ieeexplore.ieee.org/document/954909
+
+Verilog 2005:
+https://ieeexplore.ieee.org/document/1620780
+
 System Verilog 2009:
 http://ieeexplore.ieee.org/servlet/opac?punumber=5354133