Update README.md

Solidity-Expert/L2-and-Rollups/README.md

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-## Some security notes on L2s and Rollups
+# Some security notes on L2s and Rollups
 
 <br>
 
 * The current Ethereum version has low transaction throughput and high latency in processing. This means that transactions are both slow and prohibitively expensive, due to high demand, relative to what the network can take at any given time.
 
 
-### Scaling solutions
+## Scaling solutions
 
 There are two types of scaling solutions:
 
 - On-chain scaling refers to any direct modification made to a blockchain, like data sharding and execution sharding in the incoming Ethereum 2.0.
-- Off-chain scaling refers to any innovation outside of a blockchain, i.e., the execution of transaction bytecode happens externally instead of on Ethereum
+- Off-chain scaling refers to any innovation outside of a blockchain, i.e., the execution of transaction bytecode happens externally instead of on Ethereum. hese solutions are called L2, because layer 2 works above layer 1 (Ethereum) to optimize and speed up processing. Arbitrum and Optimism Ethereum are two well-known examples of L2 scaling solutions.
+
+Currently, we can distinguish between two leading L2 solutions as:
+
+- Zero-Knowledge (zk) rollups, and
+- Optimistic rollups
+
+
+### Zk-rollups
+
+* zk-rollups bundle together many off-chain transactions into a single verifiable batch using zk-SNARK. 
+* zk-SNARK is an extremely efficient, zero-knowledge proof that allows one party to prove it possesses certain information without revealing that information. These validity proofs are then posted to the Ethereum blockchain.
+
 
 ### Optimistic Rollups