Update README.md

Added Codespaces badge.

README.md

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 <img src="img/HDF5.png" alt="HDF5 logo" title="HDF5 logo" />
 
+[![Open in GitHub Codespaces](https://github.com/codespaces/badge.svg)](https://codespaces.new/HDFGroup/hdf5-tutorial)
+
 *HDF5 is a framework for sharing data.* Its purpose is to frame data so that it can be shared and understood on its own because it speaks for itself; it is *self-describing*. 'Sharing' can mean different things, and there are different ways to achieve this. The framework includes a data model and implementations such as the __[HDF5 library and file format](https://github.com/HDFGroup/hdf5)__ or the __[Highly Scalable Data Service (HSDS)](https://github.com/HDFGroup/hsds)__, and several fantastic community contributions, such as __[`h5py`](https://www.h5py.org/)__, __[HDFql](https://www.hdfql.com/)__, __[`rhdf5`](https://bioconductor.org/packages/release/bioc/html/rhdf5.html)__, or __[H5Web](https://h5web.panosc.eu/)__.
 
 This is a tutorial for new and intermediate HDF5 users. The tutorial is
@@ -18,4 +20,4 @@ The notebooks in this tutorial are:
 
 This tutorial is intended to be executed in a Web browser without installing any software. To this end, this repository comes with a prebuild __[GitHub Codespaces](https://github.com/features/codespaces)__ configuration. All you need is to launch a Codespaces environment and start evaluating the Jupyter notebooks. (You are welcome to clone this repository and set up your local environment. Notice that a few settings are specific to the Codespaces configuration and will need adjusting in a custom environment. We recommend you review [`environment.yml`](./environment.yml) for Python package dependencies, and the files in [`.devcontainer`](./.devcontainer) for additional dependencies.)
 
-Enjoy and let us know what you think about the tutorial in the __[discussion](https://forum.hdfgroup.org/)__! Make it better by reporting __[issues](https://github.com/HDFGroup/hdf5-tutorial/issues)__ or submitting __[pull requests](https://github.com/HDFGroup/hdf5-tutorial/pulls)__!
+Enjoy and let us know what you think about the tutorial in the __[discussion](https://forum.hdfgroup.org/)__! Make it better by reporting __[issues](https://github.com/HDFGroup/hdf5-tutorial/issues)__ or submitting __[pull requests](https://github.com/HDFGroup/hdf5-tutorial/pulls)__!