add build tools install instructions for CentOS

[jshimko]

Tested and confirmed working on CentOS 7.

To test:

# hop into a fresh CentOS image
docker run -it --rm centos:7 /bin/bash

# install Reaction dependencies
yum groupinstall "Development Tools"
yum install epel-release
yum install GraphicsMagick nodejs

npm i -g reaction-cli

reaction init

cd reaction 

# because we're currently root inside the container...
export METEOR_ALLOW_SUPERUSER=true

reaction

[brent-hoover]

When I follow these instructions everything works until I start reaction and then I get the following error and Reaction doesn't start

You are attempting to run Meteor as the 'root' superuser. If you are
developing, this is almost certainly *not* what you want to do and will likely
result in incorrect file permissions. However, if you are running this command
in a build process (CI, etc.), or you are absolutely sure you know what you are
doing, set the METEOR_ALLOW_SUPERUSER environment variable or pass
--allow-superuser to proceed.

Even with METEOR_ALLOW_SUPERUSER or --allow-superuser, permissions in your app
directory will be incorrect if you ever attempt to perform any Meteor tasks as
a normal user. If you need to fix your permissions, run the following command
from the root of your project:

  sudo chown -Rh <username> .meteor/local

[jshimko]

Yeah sorry, forgot to mention that. You're root by default in a container, so you need to tag on Meteor's --allow-superuser flag or run export METEOR_ALLOW_SUPERUSER=true first.

[brent-hoover]

Wouldn't we want to add that to the docs as well?

[jshimko]

No, I don't think so. There's nothing OS-specific about that warning. It happens on every Linux/Unix OS. And I think the warning itself does a pretty good job of explaining your options. Also, anyone that's developing on a Linux server should probably already be aware of the dangers of running everything as root.