Implement Πt changes, including TODO updates and feature additions.

This change introduces Πt implementation, a differentially anonymous mixnet architecture, and outlines its performance exploration. The TODO list has been updated to include several feature additions and enhancements: client-side encryption of nonces into headers, calculation of checkpoint onions, expected nonce numbers for each layer, and time windows for onion arrival at each hop. The Node features have been expanded to include checking for onion lateness or mismatched nonces, resulting in the addition of bruises. Additionally, the README.md file now includes an introduction to this project, providing context for the Πt exploration.

README.md

@@ -1,6 +1,16 @@
 Implementing &Pi;<sub>t</sub>
 ================
 
+
+**TODO**
+- Client: Encrypt nonces into header
+- Client: use prf to calculate when to generate checkpoint onions (when forming onion)
+- Client: calculate how many checkpoint onions each node on path should expect
+- Client calculate time window for when onion should arrive at each hop
+- Node: calculate expected number of nonces for each layer
+- Node: check if onion is late or the nonce is not in expected set, add bruises if so
+- 
+
 ## Introduction
 
 This project focuses on implementing &Pi;<sub>t</sub>, a differentially anonymous mixnet architecture, to explore its performance under