Create CITATION.md

CITATION.md

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+Citing This Library
+-------------------
+
+`galgebra` is published to zenodo.
+DOI [10.5281/zenodo.10902114](https://doi.org/10.5281/zenodo.10902114) refers to all versions of `galgebra`.
+
+If you want to cite all releases, use:
+```BibTeX
+@software{galgebra,
+  author       = {Alan Bromborsky and
+                  Utensil Song and
+                  Eric Wieser and
+                  Hugo Hadfield and
+                  {The Pygae Team}},
+  title        = {pygae/galgebra},
+  month        = jun,
+  year         = 2020,
+  publisher    = {Zenodo},
+  doi          = {10.5281/zenodo.3857096},
+  url          = {https://doi.org/10.5281/zenodo.3857096}
+}
+```
+
+To obtain BibTex citation information for a _specific_ release (recommended):
+
+* Run `python -m pip show galgebra` to determine which version you are using (or print `galgebra.__version__` from python)
+* Click on the corresponding version from [this list of versions](https://zenodo.org/search?q=parent.id%3A3857096&f=allversions%3Atrue&l=list&p=1&s=10&sort=version)
+* Scroll down to the bottom of the page, and click the "BibTex" link in the "Export" sidebar