Update README.md by adding new paper info

[zhenyuz0500]

Adding three papers, including CausalML whitepaper.

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[paullo0106]

Thanks @zhenyuz0500 !
I think the order might need to be adjusted a bit? Because in the The package currently supports the following methods description section, each method is associated to the corresponding paper number in the list. For example, Uplift tree/random forests should be linked to [2] after the changes.

[paullo0106]

Thanks!