docs(readme): initial readme

Author: grantila

README.md

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+[![npm version][npm-image]][npm-url]
+[![downloads][downloads-image]][npm-url]
+[![build status][build-image]][build-url]
+[![coverage status][coverage-image]][coverage-url]
+[![Language grade: JavaScript][lgtm-image]][lgtm-url]
+[![Node.JS version][node-version]][node-url]
+
+
+# rotated-array-set
+
+`RotatedArraySet` is a class looking a bit like the built-in [Set](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Set) but is a set of arrays of `T`, and treats rotated arrays as "same".
+
+
+# API
+
+Construct a `RotatedArraySet`, optionally provide a *stringify* method, converting `T` to `string` (this is not necessary for primitive types).
+
+
+## Example
+
+```ts
+import { RotatedArraySet } from 'rotated-array-set'
+
+const tree = new RotatedArraySet< string >( );
+
+tree.insert( [ 'a', 'b', 'c' ] );
+tree.insert( [ 'x', 'y' ] );
+tree.insert( [ 'c', 'a', 'b' ] ); // won't insert, already has this but rotated
+tree.insert( [ 'y', 'x' ] ); // won't insert, same reason
+
+tree.has( [ 'b', 'c', 'a' ] ); // true
+tree.has( [ 'c', 'b', 'a' ] ); // false - this isn't *rotated*
+
+tree.values( ); // [ [ 'a', 'b', 'c' ], [ 'x', 'y' ] ]
+```
+
+Provide a custom stringifier:
+
+```ts
+import { RotatedArraySet } from 'rotated-array-set'
+
+const tree = new RotatedArraySet< User >( user => `${user.first} ${user.last}` );
+
+tree.insert( [ user1, user2, user3 ] );
+tree.insert( [ user3, user1, user2 ] ); // won't insert, already has this but rotated
+
+tree.has( [ user2, user3, user1 ] ); // true
+tree.has( [ user3, user2, user1 ] ); // false - not *rotated*
+
+tree.values( ); // [ [ user1, user2, user3 ] ]
+```
+
+
+[npm-image]: https://img.shields.io/npm/v/rotated-array-set.svg
+[npm-url]: https://npmjs.org/package/rotated-array-set
+[downloads-image]: https://img.shields.io/npm/dm/rotated-array-set.svg
+[build-image]: https://img.shields.io/github/workflow/status/grantila/rotated-array-set/Master.svg
+[build-url]: https://github.com/grantila/rotated-array-set/actions?query=workflow%3AMaster
+[coverage-image]: https://coveralls.io/repos/github/grantila/rotated-array-set/badge.svg?branch=master
+[coverage-url]: https://coveralls.io/github/grantila/rotated-array-set?branch=master
+[lgtm-image]: https://img.shields.io/lgtm/grade/javascript/g/grantila/rotated-array-set.svg?logo=lgtm&logoWidth=18
+[lgtm-url]: https://lgtm.com/projects/g/grantila/rotated-array-set/context:javascript
+[node-version]: https://img.shields.io/node/v/rotated-array-set
+[node-url]: https://nodejs.org/en/