Create on_tx_hook.md (#2023)

design/ux/on_tx_hook.md

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+# Platform-level traffic incentives 
+
+We want a flexible mechanism to incentivise traffic.
+
+For example:
+- every transaction is rewarded with a ZEN token
+- if you hold over 1000 TEN tokens, you receive two ZEN tokens
+- if you use the newly launched app X during the first week, you double your reward
+- randomly, tx senders could receive hidden prizes
+- every 1000 transactions, you receive 100 ZEN
+- etc
+
+## Solution
+
+In the Genesis block, we  deploy an upgradeable contract. The address is saved.
+This contract will have a method `onTransactions(tx[])` that delegates to a dynamic implementation.
+
+*Note1: The method receives all transactions from the batch.*
+*Note2: The `tx` - is a reduced object containing everything except the calldata*
+
+
+At the end of every batch, our EVM implementation will create a synthetic transaction that calls `onTransactions`.
+
+
+## Discusson
+
+- the logic should be simple because it will be applied to every transaction, and it will impact performance.
+- The implementation can be changed at any time
+- The contract should not fail.
+-