Add HardNet8v2 citation (arXiv 2007.09699)

README.md

@@ -35,4 +35,11 @@ hard-negative mining triplet loss. NeurIPS 2017.
     booktitle = {NeurIPS},
     year      = {2017}
 }
+
+@article{HardNet2020,
+    title   = {Improving the HardNet Descriptor},
+    author  = {Milan Pultar},
+    journal = {arXiv ePrint 2007.09699},
+    year    = {2020}
+}
 ```