Add Homebrew build instructions

BUILD.md

diff --git a/BUILD.md b/BUILD.md
index 7f4d4bc..e9d7d4d 100644
--- a/BUILD.md
+++ b/BUILD.md
@@ -6,9 +6,12 @@
 whereas on Fedora, RHEL, CentOS, and other Red Hat based systems you would do:
 * sudo yum install gcc gcc-c++ freetype-devel glib2-devel cairo-devel
 
-or using MacPorts:
+on the Mac, using MacPorts:
 * sudo port install freetype glib2 cairo
 
+or using Homebrew:
+* brew install freetype glib cairo
+
 If you are using a tarball, you can now proceed to running configure and make
 as with any other standard package. That should leave you with a shared
 library in src/, and a few utility programs including hb-view and hb-shape
@@ -21,5 +24,8 @@
 and on Fedora, RHEL, CentOS:
 * sudo yum install autoconf automake libtool pkgconfig ragel gtk-doc
 
-or using MacPorts:
+on the Mac, using MacPorts:
 * sudo port install autoconf automake libtool pkgconfig ragel gtk-doc
+
+or using Homebrew:
+* brew port install autoconf automake libtool pkgconfig ragel gtk-doc