Add README for github repo.

README.md

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+Spack
+===========
+
+Spack is a package management tool designed to support multiple
+versions and configurations of software on a wide variety of platforms
+and environments. It was designed for large supercomputing centers,
+where many users and application teams share common installations of
+software on clusters with exotic architectures, using libraries that
+do not have a standard ABI. Spack is non-destructive: installing a new
+version does not break existing installations, so many configurations
+can coexist on the same system.
+
+Most importantly, Spack is simple. It offers a simple spec syntax so
+that users can specify versions and configuration options
+concisely. Spack is also simple for package authors: package files are
+writtin in pure Python, and specs allow package authors to write a
+single build script for many different builds of the same package.
+
+See the
+[Feature Overview](http://scalability-llnl.github.io/spack/features.html)
+for examples and highlights.
+
+To install spack and install your first package:
+
+    $ git clone git@github.com:scalability-llnl/spack.git
+    $ cd spack/bin
+    $ ./spack install libelf
+
+Documentation
+----------------
+
+[Full documentation](http://scalability-llnl.github.io/spack)
+for Spack is also available.
+
+Authors
+----------------
+Spack was written by Todd Gamblin, tgamblin@llnl.gov.