update README (#366)

Author: adekbadek

README.md

@@ -119,6 +119,8 @@ See package.json and Gemfile for versions
 1. Check that you're using the right version of node. Run `nvm list` to check. Use 5.5 or greater.
 1. Check that you have Postgres installed. Run `which postgres` to check. Use 9.4 or greater.
 1. Check that you have `qmake` installed. Run `which qmake` to check. If missing, follow these instructions: [Installing Qt and compiling capybara-webkit](https://github.com/thoughtbot/capybara-webkit/wiki/Installing-Qt-and-compiling-capybara-webkit)
+1. Check that you have Redis installed. Run `which redis-server` to check. If missing, install with Homebrew (`brew install redis`) or follow [these instructions](https://redis.io/topics/quickstart#installing-redis
+  ).
 1. `bundle install`
 1. `brew install yarn`
 1. `yarn`