Update Brewfile & INSTALL instructions 4 using it

 [ci skip]

Author: zbeekman

Brewfile

@@ -10,12 +10,11 @@
 # Once Homebrew/Linuxbrew is installed, execute the following commands to install the required prerequisites:
 # `brew tap homebrew/bundle && brew bundle`
 #
-# NOTE: Until Linuxbrew upgrades to GCC 6.1 or later, you will have to edit the `mpich` line to:
-#    `brew 'mpich', args: ['cc=gcc-5', 'cxx=g++-5']`
 
 brew 'cmake'
 brew 'gcc'
 brew 'mpich', args: ['cc=gcc-6', 'cxx=g++-6']
+brew 'opencoarrays'
 
 # [Homebrew]: http://brew.sh
 # [Linuxbrew]: http://linuxbrew.sh

INSTALL.md

@@ -43,12 +43,25 @@ the OpenCoarrays installation script.
 OS X users may use the [Homebrew] or [MacPorts] package management systems to install OpenCoarrays.
 We recommend [Homebrew].
 
-Homebrew installation steps:
+Basic Homebrew installation steps:
 ```
 brew update
 brew install opencoarrays
 ```
 
+OpenCoarrays also ships with a
+[`Brewfile`](https://github.com/sourceryinstitute/opencoarrays/blob/master/Brewfile)
+that will make it easier to install opencoarrays using MPICH built
+with GCC. To install using the
+[`Brewfile`](https://github.com/sourceryinstitute/opencoarrays/blob/master/Brewfile)
+with MPICH wrapping GCC, follow these steps:
+
+```
+brew tap homebrew/bundle
+brew update
+brew bundle
+```
+
 MacPorts installation steps:
 ```
 sudo port selfupdate