[added] add readme

README.md

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+# koa-generic-session-rethinkdb
+
+This is a module for storing sessions from the [koa-generic-session](https://github.com/koajs/generic-session)
+module in [RethinkDB](http://rethinkdb.com).
+
+
+## Installation
+
+```bash
+npm i --save koa-generic-session-rethinkdb
+```
+
+## Usage
+
+### Example
+```JavaScript
+var koa = require('koa')
+// for use with koa-generic-session
+var session = require('koa-generic-session')
+var RethinkSession = require('koa-generic-session-rethinkdb')
+var rethinkdb = require('rethinkdbdash')
+
+var connection = rethinkdb({
+  host: 'localhost',
+  port: 28015
+})
+
+var sessionStore = new RethinkSession({connection: connection})
+// create the db, table and indexes to store sessions
+sessionStore.setup()
+
+
+
+var app = koa()
+// used for cookie stuffs
+app.keys = ['foo', 'bar']
+
+app.use(session({
+  store: sessionStore
+})
+```
+
+### `new RethinkSession(opts)`
+Return a new RethinkSession store. `opts` are options.
+
+#### Options
+
+* `connection` - a [rethinkdbdash](https://github.com/neumino/rethinkdbdash)
+  connection to a rethink server or cluster. required.
+* `dbName` - the name of a db to connnect to or create. optional.
+* `tableName` - the name of the table to store session in. optional.
+
+
+## Contributing
+
+Run the tests with `npm test`. Please add tests to cover new functionality.