docs: add README

README.md

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+# typst-undergradmath
+
+<p align="center">
+  <a href="http://creativecommons.org/licenses/by-sa/4.0/">
+    <img alt="CC BY-SA 4.0" src="https://img.shields.io/badge/License-CC%20BY--SA%204.0-lightgrey.svg" />
+  </a>
+</p>
+
+A Typst port of [undergradmath]
+
+## License
+
+[Like undergradmath], typst-undergradmath is licensed under the
+[Creative Commons Attribution-ShareAlike 4.0 International License][cc-by-sa].
+
+[![CC BY-SA 4.0][cc-by-sa-image]][cc-by-sa]
+
+[cc-by-sa]: http://creativecommons.org/licenses/by-sa/4.0/
+[cc-by-sa-image]: https://licensebuttons.net/l/by-sa/4.0/88x31.png
+[cc-by-sa-shield]: https://img.shields.io/badge/License-CC%20BY--SA%204.0-lightgrey.svg
+
+[undergradmath]: https://gitlab.com/jim.hefferon/undergradmath
+[Like undergradmath]: https://gitlab.com/jim.hefferon/undergradmath/-/blob/5b19eff74454f7c71664f85e8042d7b30fcf9cfb/LICENSE