Add a basic README for the refcard

Author: bbatsov

refcard/README.md

@@ -0,0 +1,12 @@
+# CIDER Quick Reference
+
+CIDER's quick reference is a single-page pdf containing the essential commands one needs to know
+to use CIDER effectively. You're encouraged to print it, laminate it and keep it on your desk.
+
+If you want to make changes to it you should first edit `cider-refcard.tex` and then run the following command
+to regenerate the pdf file:
+
+    $ pdflatex cider-refcard.tex
+
+Note that you might have to install `pdflatex` first. You can find instructions for all major operating
+systems [here](https://www.latex-project.org/get/).