Add xxhash

third_party/xxhash/doc/README.md

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+xxHash Specification
+=======================
+
+This directory contains material defining the xxHash algorithm.
+It's described in [this specification document](xxhash_spec.md).
+
+The algorithm is also be illustrated by a [simple educational library](https://github.com/easyaspi314/xxhash-clean),
+written by @easyaspi314 and designed for readability
+(as opposed to the reference library which is designed for speed).