Update README.md

[Redis](http://redis.io/) (must be version 2.6.12 or later).

These should be running on the host system.

By default the ShareLaTeX Docker container looks for these running on the host

machine at port 27017 (for Mongo) and port 6379 (for Redis). These are the defaults

ports for both databases so you shouldn't need to change them.

Note that Docker containers each come with their own network stack, and Mongo and Redis

often listen by default on `127.0.0.1` which is not accessible on the host

from inside the Docker container. Instead, you should configure Mongo and Redis to

also listen on `172.17.42.1` (or whatever ip iddress the docker0 interface has on your

host). This can be done in `/etc/mongod.conf` and `/etc/redis/redis.conf`.

```

# /etc/mongod.conf

...

bind_ip = 172.17.42.1

...

```

```

# /etc/redis/redis.conf

...

bind 172.17.42.1

...

```

By default the ShareLaTeX Docker container looks for these running on the host

machine at port 27017 (for Mongo) and port 6379 (for Redis). These are the defaults

ports for both databases so you shouldn't need to change them.

If you want to point ShareLaTeX at a database in a different location, you can

configure the container with environment variables. See the **Configuration Options**

section below.