Convert README to md (#777)

Author: DeaMariaLeon

README.md

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+![Sparse](docs/assets/images/logo_with_text.svg)
+
+# Sparse Multidimensional Arrays
+
+[![Build Status](https://github.com/pydata/sparse/actions/workflows/ci.yml/badge.svg)](
+  https://github.com/pydata/sparse/actions/workflows/ci.yml)
+[![Docs Status](https://readthedocs.org/projects/sparse-nd/badge/?version=latest)](
+  http://sparse.pydata.org/en/latest/?badge=latest)
+[![Coverage](https://codecov.io/gh/pydata/sparse/branch/main/graph/badge.svg)](
+  https://codecov.io/gh/pydata/sparse)
+
+## This library provides multi-dimensional sparse arrays.
+
+- 📚 [Documentation](http://sparse.pydata.org)
+
+- 🙌 [Contributing](https://github.com/pydata/sparse/blob/main/docs/contributing.md)
+
+- 🪲 [Bug Reports/Feature Requests](https://github.com/pydata/sparse/issues)