Add initial org-level README file

README.md

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+# Welcome to Google Quantum AI's open-source projects
+
+Quantum AI and collaborators from all around the world maintain and develop
+many open-source software projects on GitHub. We build best-in-class libraries
+and utilities to advance applications research and error-correction research
+for the future of fault-tolerant quantum computing.
+
+## Repositories
+
+Quantum AI's open-source projects are found in repositories under the
+Quantumlib organization and elsewhere on GitHub. Our major efforts include the
+following:
+
+*   [Cirq](https://github.com/quantumlib/Cirq): A Python framework for
+    creating, editing, manipulating, and running quantum circuits on hardware
+    or in simulation.
+
+*   [OpenFermion](https://github.com/quantumlib/OpenFermion): An open-source
+    library for compiling and translating problems in chemistry and materials
+    science into quantum circuits.
+
+*   [Qualtran](https://github.com/quantumlib/Qualtran): A library of
+    blueprinted fault-tolerant algorithms and advanced resource estimation
+    tools used to cost out physical resource requirements.
+
+*   [qsim](https://github.com/quantumlib/qsim): A fast Schrödinger state-vector
+    simulator written in C++  and Python that integrates with Cirq to handle
+    large-scale classical simulations using vector instructions.
+
+*   [Stim](https://github.com/quantumlib/Stim): A high-performance, fast
+    stabilizer circuit simulator optimized for analyzing quantum stabilizer
+    circuits and stabilizer-based error-correcting codes.
+
+*   [TensorFlow Quantum](https://github.com/tensorflow/quantum): An open-source
+    framework for hybrid quantum-classical machine learning.
+
+Other projects include:
+
+*   [Chromobius](https://github.com/quantumlib/Chromobius): A fast
+    implementation of the Möbius color-code decoder that approximates color
+    code decoding as a minimum weight matching problem.
+
+*   [ReCirq](https://github.com/quantumlib/ReCirq): A repository containing
+    research examples and published experiments leveraging Cirq.
+
+*   [Tesseract Decoder](https://github.com/quantumlib/tesseract-decoder): A
+    specialized decoder built for quantum error correction research.
+
+*   [TypedUnits](https://github.com/quantumlib/TypedUnits): A specialized
+    library for managing units of measurement.
+
+*   [Unitary](https://github.com/quantumlib/unitary): A library exploring
+    quantum computing concepts through games.
+
+## Get started with Quantum AI open-source software!
+
+We welcome contributions from the quantum research community! Everyone is
+invited to participate in our projects. To get up to speed quickly, explore our
+core software offerings below and refer to their respective README files and
+other documentation.