Give each template its own README

The root README keeps what is shared — dependencies, accessibility, the
scripts — and links to a per-template README for how to build and what is
in it. The Posters section moves to poster_template/README.md with a
pointer left behind, since it is that template's caveat.

slides_template/README.md replaces the two READMEs that came with
beamer-mako.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

memo_template/README.md

@@ -0,0 +1,18 @@
+# Memo Template
+
+A short memo in the standard `article` class, styled by `cdsc-memo.sty`. The
+title and author sit on one line above the text rather than in a title block.
+See the [root README](../README.md) for tagging and accessibility.
+
+```
+make            # build the PDF
+make vc         # write a vc file with the current git revision
+make clean
+```
+
+Fonts are Libertinus Serif and TeX Gyre Heros, from `texlive-fonts-extra`;
+`text.tex` has a commented alternative for Adobe Garamond Premier Pro.
+Bibliography is biblatex with biber, APA style, reading `refs.bib`.
+
+Uncomment the `\input{vc}` and `\pagestyle{cdsc-page-memo-git}` lines in
+`text.tex` to print the revision at the foot of each page.