Add citation.

README.md

@@ -76,4 +76,18 @@ Implementing new query parsers is a little more tricky than new SERP renderers.
 
 ## License
 
-MIT
+MIT
+
+## Citing
+
+If you use this library in your research, please cite [the following paper](https://dl.acm.org/doi/10.1145/3539618.3591819):
+
+```
+@inproceedings{scells2023pyboolir,
+    author = {Scells, Harrisen and Potthast, Martin},
+    title = {Pybool_ir: A Toolkit for Domain-Specific Search Experiments},
+    year = {2023},
+    booktitle = {Proceedings of the 46th International ACM SIGIR Conference on Research and Development in Information Retrieval},
+    pages = {3190–3194},
+}
+```