Add educational decoder to /contrib

contrib/educational_decoder/README.md

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+Educational Decoder
+===================
+
+`zstd_decompress.c` is a self-contained implementation of a decoder according
+to the Zstandard format specification written in C99.
+While it does not implement as many features as the reference decoder,
+such as the streaming API or content checksums, it is written to be easy to
+follow and understand, to help understand how the Zstandard format works.
+It's laid out to match the [format specification],
+so it can be used to understand how confusing segments could be implemented.
+It also contains implementations of Huffman and FSE table decoding.
+
+[format specification]: https://github.com/facebook/zstd/blob/dev/doc/zstd_compression_format.md
+
+`harness.c` provides a simple test harness around the decoder:
+
+    harness <input-file> <output-file> [dictionary]
+