Add a Brewfile to install prereqs with Homebrew

 Homebrew users can now get all the prereqs easily installed:

 ```
 brew tap homebrew/bundle
 brew bundle
 ```

 [ci skip]

Author: zbeekman

Brewfile

@@ -0,0 +1,21 @@
+# Brewfile to install OpenCoarrays prerequisites with [Homebrew] or [Linuxbrew].
+#
+# TL;DR: `brew tap homebrew/bundle && brew bundle` to install prerequisites
+#
+# Homebrew for Mac OS X can be installed via:
+# `/usr/bin/ruby -e "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/install)"`
+# Linux brew (lives in userland, no sudo required) can be installed with:
+# `ruby -e "$(curl -fsSL https://raw.githubusercontent.com/Linuxbrew/linuxbrew/go/install)"`
+#
+# 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']
+
+# [Homebrew]: http://brew.sh
+# [Linuxbrew]: http://linuxbrew.sh