add readme

examples/tx-observability/README.md

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+# tx observability
+
+Opt into a standardized set of events ([TBD](https://github.com/flashbots/suave-std/pull/85)) to enable interoperability with other SUAPPS, block-builders, L2 networks, etc.
+
+Events are defined in an abstract contract (ideally in suave-std):
+
+```solidity
+abstract contract ObservableOrderflow {
+    event SentTransaction(bytes32 txHash);
+}
+```
+
+Then SUAPP developers import and emit those events in their own callbacks:
+
+```solidity
+contract Suapp is ObservableOrderflow {
+    event MySuappEvent(uint256 x);
+
+    modifier confidential() {
+        require(Suave.isConfidential(), "must be called confidentially");
+        _;
+    }
+
+    function didSomething(bytes32[] memory txHashes, uint256 x) public confidential {
+        emit MySuappEvent(x);
+        emit SentTransactions(txHashes);
+    }
+
+    function doSomething() public confidential returns (bytes memory) {
+        // pretend these are tx hashes that we're handling in our SUAPP
+        bytes32[] memory txHashes = new bytes32[](3);
+        for (uint256 i = 0; i < txHashes.length; i++) {
+            txHashes[i] = keccak256(abi.encode("tx", i));
+        }
+        return abi.encodeWithSelector(this.didSomethingWithTxs.selector, txHashes, 9001);
+    }
+}
+```