Add missing cheatsheets

Author: josevalim

public/downloads/cheatsheets/README.md

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+This directory contains TeX source files and their corresponding
+PDF files, containing small help pages, reminders, compressed
+references, also called cheat sheets.
+
+To produce a .pdf starting from the .tex source, you need `tex`,
+and more specifically [LaTeX](https://www.latex-project.org/get/).
+
+The BasicTex variant can be used as long as the following packages
+are available:
+
+    $ tlmgr install courier framed charter enumitem ec helvetica
+
+To compile a tex file EQUIS.tex into its corresponding EQUIS.pdf:
+
+    $ texi2pdf EQUIS.tex
+
+You may ignore all output except the very last two lines, which
+should look like:
+
+    Output written on EQUIS.pdf (2 pages, 69846 bytes).
+    Transcript written on EQUIS.log.