update (main) readme

Author: serial-coder

Diff for: README.md

@@ -16,6 +16,7 @@ Nonetheless, feel free to contact **[Valix Consulting](https://valix.io)** for y
 | 02 | [Reentrancy](02_reentrancy) | - [On Medium](https://medium.com/valixconsulting/solidity-smart-contract-security-by-example-02-reentrancy-b0c08cfcd555)<br /> - [On serial-coder.com](https://www.serial-coder.com/post/solidity-smart-contract-security-by-example-02-reentrancy/) |
 | 03 | [Reentrancy via Modifier](03_reentrancy_via_modifier) | - [On Medium](https://medium.com/valixconsulting/solidity-smart-contract-security-by-example-03-reentrancy-via-modifier-fba6b1d8ff81)<br /> - [On serial-coder.com](https://www.serial-coder.com/post/solidity-smart-contract-security-by-example-03-reentrancy-via-modifier/) |
 | 04 | [Cross-Function Reentrancy](04_cross_function_reentrancy) | - [On Medium](https://medium.com/valixconsulting/solidity-smart-contract-security-by-example-04-cross-function-reentrancy-de9cbce0558e)<br /> - On serial-coder[.]com (coming soon) |
+| 05 | [Cross-Contract Reentrancy](04_cross_contract_reentrancy) | - On Medium (coming soon)<br /> - On serial-coder[.]com (coming soon) |
 
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