Add README to nearest neighbor

Author: bytesnake

algorithms/linfa-nn/README.md

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+# Nearest Neighbor
+
+`linfa-nn` provides a pure Rust implementation of nearest neighbor algorithms.
+
+## The Big Picture
+
+`linfa-nn` is a crate in the [`linfa`](https://crates.io/crates/linfa) ecosystem, an effort to create a toolkit for classical Machine Learning implemented in pure Rust, akin to Python's `scikit-learn`.
+
+Nearest neighbor search (NNS), as a form of proximity search, is the optimization problem of finding the point in a given set that is closest (or most similar) to a given point. Closeness is typically expressed in terms of a dissimilarity function: the less similar the objects, the larger the function values. 
+
+## Current State
+
+ linfa-nn currently provides the following implementations:
+    - linear
+    - balltree
+    - KDTree
+
+
+## License
+Dual-licensed to be compatible with the Rust project.
+
+Licensed under the Apache License, Version 2.0 <http://www.apache.org/licenses/LICENSE-2.0> or the MIT license <http://opensource.org/licenses/MIT>, at your option. This file may not be copied, modified, or distributed except according to those terms.