feat: adding BibTex entry to cite Substrait (#481)

Closes #480

CITATION.cff

@@ -0,0 +1,17 @@
+cff-version: 1.2.0
+title: >-
+  Substrait: Cross-Language Serialization for Relational
+  Algebra
+message: >-
+  If you use this software, please cite it using the
+  metadata from this file.
+type: software
+authors:
+  - given-names: substrait-io
+identifiers:
+  - type: url
+    value: 'https://github.com/substrait-io/substrait'
+repository-code: 'https://github.com/substrait-io/substrait'
+url: 'https://substrait.io/'
+license: Apache-2.0
+date-released: '2021-09-01'

site/docs/community/index.md

@@ -31,6 +31,23 @@ Want to learn more about Substrait? Try the following presentations and slide de
 
 * Substrait: A Common Representation for Data Compute Plans (Jacques Nadeau, April 2022) [[slides](https://docs.google.com/presentation/d/1H89iwnnmHZ2oMgGpFon9Rv_fJ60RWE0c9drHCZAL8Bw)]
 
+## Citation
+
+If you use Substrait in your research, please cite it using the following BibTeX entry:
+
+```bibtex
+@misc{substrait,
+  author = {substrait-io},
+  title = {Substrait: Cross-Language Serialization for Relational Algebra},
+  year = {2021},
+  month = {8},
+  day = {31},
+  publisher = {GitHub},
+  journal = {GitHub repository},
+  howpublished = {\url{https://github.com/substrait-io/substrait}}
+}
+```
+
 ## Contribution
 
 All contributors are welcome to Substrait.