Merge #329

329: Create CITATION.bib (as per #327) r=eldruin a=cvvletter

which allows for a pre-made Github citation button.

Fixes #327 

Co-authored-by: cvvletter <cvvletter@pm.me>

Author: bors[bot]

CITATION.bib

@@ -0,0 +1,7 @@
+@booklet{embedded-rust-book,
+  title = {The Embedded Rust Book},
+  author = {{Rust on Embedded Devices Working Group} and others},
+  date = {}, % intentionally left empty because the book has no 'publication date'
+  url = {https://docs.rust-embedded.org/book/},
+  urldate = {}, % fill this in manually
+}