Add four links to dicussion groups on taffic-obf.

Thanks to David Fifield for the summary and the links.

references.bib

@@ -1086,6 +1086,7 @@ @article{Bocovich2018a
 	year = {2018},
 	pages = {43--62},
 	url = {https://www.petsymposium.org/2018/files/papers/issue3/popets-2018-0020.pdf},
+	discussion_url = {https://groups.google.com/g/traffic-obf/c/76xh3oS6wkE/m/pUroVpICGAAJ},
 }
 
 @inproceedings{Barradas2018a,
@@ -1095,6 +1096,7 @@ @inproceedings{Barradas2018a
 	publisher = {USENIX},
 	year = {2018},
 	url = {https://www.usenix.org/system/files/conference/usenixsecurity18/sec18-barradas.pdf},
+	discussion_url = {https://groups.google.com/g/traffic-obf/c/qAFtEo2pj1M/m/uZDLGYhjBgAJ},
 }
 
 @inproceedings{VanderSloot2018a,
@@ -1133,6 +1135,7 @@ @inproceedings{Manfredi2018a
 	publisher = {USENIX},
 	year = {2018},
 	url = {https://www.usenix.org/system/files/conference/foci18/foci18-paper-manfredi.pdf},
+	discussion_url = {https://groups.google.com/g/traffic-obf/c/X6fE99hinMU/m/OwbH7TldBgAJ},
 }
 
 @inproceedings{Knockel2018a,
@@ -1369,6 +1372,7 @@ @inproceedings{Jermyn2017a
 	publisher = {USENIX},
 	year = {2017},
 	url = {https://www.usenix.org/system/files/conference/foci17/foci17-paper-jermyn.pdf},
+	discussion_url = {https://groups.google.com/g/traffic-obf/c/HkmVtobDHmM/m/UNjeyVujCAAJ},
 }
 
 @inproceedings{Frolov2017a,