README: citing guidelines

Author: nschneid

README.md

@@ -5,3 +5,17 @@ This organization contains corpora annotated according to the [UCCA scheme](http
 
 The repository contains documentation related to the annotation, including file formats and annotation guidelines.
 
+The [guidelines](guidelines.pdf) should be cited as:
+
+> Omri Abend, Nathan Schneider, Dotan Dvir, Jakob Prange, and Ari Rappoport (Dec. 2020). UCCA's Foundational Layer: Annotation Guidelines v2.1. arXiv preprint. http://arxiv.org/abs/2012.15810
+
+```bib
+@article{abend-20,
+	title = {{UCCA's} {F}oundational {L}ayer: {A}nnotation Guidelines v2.1},
+	url = {http://arxiv.org/abs/2012.15810},
+	journal = {{arXiv}:2012.15810 [cs]},
+	author = {Abend, Omri and Schneider, Nathan and Dvir, Dotan and Prange, Jakob and Rappoport, Ari},
+	month = dec,
+	year = {2020}
+}
+```