Add BUILD.md based on harfbuzz.org docs

BUILD.md

+On Linux, install the development packages for
+[FreeType](http://www.freedesktop.org/wiki/Software/FreeType/),
+Cairo, and GLib. For example, on Ubuntu / Debian, you would do:
+* sudo apt-get install gcc g++ libfreetype6-dev libglib2.0-dev libcairo2-dev
+
+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:
+* sudo port install freetype glib2 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
+under util/.
+If you are bootstraping from git, you need a few more tools before you can
+run autogen.sh for the first time. Namely, pkg-config and ragel. Again,
+on Ubuntu / Debian:
+* sudo apt-get install autoconf automake libtool pkg-config ragel gtk-doc-tools
+
+and on Fedora, RHEL, CentOS:
+* sudo yum install autoconf automake libtool pkgconfig ragel gtk-doc
+
+or using MacPorts:
+* sudo port install autoconf automake libtool pkgconfig ragel gtk-doc