Add archive announcement

Author: youknowone

README.md

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 # malachite-bigint
 
+ANNOUNCEMENT: This project is now integrated in the [malachite](https://github.com/mhogrefe/malachite) project. Please visit malachite for recent versions and releases.
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 A drop-in replacement for rust [num-bigint](https://github.com/rust-num/num-bigint) based on [malachite](https://github.com/mhogrefe/malachite) `Natural`.
 
 This is not an official malachite project. Please directly use malachite unless you need num-bigint compatibility.