TWP: Extend the discussion of scalability.

docs/source/whitepaper/Ref.bib

@@ -398,4 +398,11 @@ publisher = {USENIX Association},
     author = {Ittai Anati and Shay Gueron and Simon P Johnson and Vincent R Scarlata},
     title = {Innovative Technology for CPU Based Attestation and Sealing},
     year = {2013}
+}
+
+@misc{PayPalTrafficVolume,
+    author = {Craig Smith},
+    title = {100 Amazing PayPal Statistics and Facts (2019)},
+    year = {2019},
+    howpublished = {\url{https://expandedramblings.com/index.php/paypal-statistics/}}
 }