Add initial README

README.md

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+This is a Nim wrapper for https://github.com/prysmaticlabs/hashtree which is
+about SHA256 hashing batches of 64B blocks in parallel which can be useful to
+hash a [Merkle Tree](https://en.wikipedia.org/wiki/Merkle_tree).
+
+## Building & Testing
+
+It should just work to:
+
+    nimble install sha256_64B
+
+You could then do an out-of-tree test by
+```
+    cp -pr tests /tmp
+    cd /tmp/tests
+    nim c bench
+    ./bench a28
+```
+(On Windows you may need to use a very large `-b=` to get a long enough test
+for meaningful times.)
+
+## Supported platforms
+
+The optimization so far is only done on amd64/x86\_64 and some ARM. Object files
+for libhashtree.a are only provided for Linux, Windows, & OSX.  (This should be
+made more specific in a few ways.)
+
+## License
+
+Licensed and distributed under either of
+
+* MIT license: [LICENSE-MIT](LICENSE-MIT) or http://opensource.org/licenses/MIT
+
+or
+
+* Apache License, Version 2.0, ([LICENSE-APACHEv2](LICENSE-APACHEv2) or http://www.apache.org/licenses/LICENSE-2.0)
+
+at your option. These files may not be copied, modified, or distributed except according to those terms.