Rewrite the port's README as Markdown

Converts it and fills in what was missing: install instructions, links to
ltx-talk, trantor, veraPDF and the presenter viewers, the upstream issues
behind the speaker-notes workaround and the overlay tagging bug, and how to
use the notes machinery in another document.

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

slides_template/README.ltx-talk.md

@@ -0,0 +1,103 @@
+# ltx-talk port
+
+This branch is the presentation template ported from beamer to the
+[ltx-talk](https://github.com/josephwright/ltx-talk) class. ltx-talk was
+written with tagging as a design goal.
+
+## Requirements
+
+  * LuaLaTeX
+  * TeX Live 2025 or newer (ltx-talk needs LaTeX 2025-11-01 or later);
+    `texlive-latex-recommended` ships the class
+  * `python3-pypdf`, for assembling the notes PDF
+
+You can install all three on Debian with:
+
+```
+apt install texlive-latex-recommended python3-pypdf
+```
+
+## Building
+
+```
+make slides      # the deck; this is what gets uploaded
+make notes       # double-width slide + notes pages for presenting
+make pdfpc       # build the notes PDF and open it in pdfpc
+```
+
+The notes are a kludge. They exist to be read while talking and not shared.
+Only the slides build is expected to validate with tools like
+[veraPDF](https://verapdf.org/).
+
+## Speaker notes
+
+ltx-talk has no `\note` command. Its README and website list speaker notes
+among what the class is for, but the feature is unwritten (see
+[issue #156](https://github.com/josephwright/ltx-talk/issues/156)).
+
+`talk-notes.sty` defines `\note` so that it records its text to a side file and
+contributes nothing to the slides. `mknotes` reads that file afterwards and
+builds the presenter PDF.
+
+The assembled page puts the slide on the left and, on the right, a grey band
+carrying the section and position in the talk, a thumbnail of the current
+slide flush into the corner, and the notes below. Previous and next slides are
+left to the viewer: [dspdfviewer](https://github.com/dannyedel/dspdfviewer),
+[pdfpc](https://pdfpc.github.io/), or the web viewers
+[Beamer Viewer](https://beamerviewer.euxane.eu/) and
+[backstage](https://bckstg.xyz/).
+
+### Installing speaker notes
+
+To use speaker notes in another ltx-talk document, copy two files out of this
+repository:
+
+- `mknotes`
+- `talk-notes.sty`
+
+Load the package in the preamble:
+
+```latex
+\usepackage{talk-notes}
+```
+
+Write notes with `\note{...}`, or `\note<2->{...}` to bind one to particular
+slides of a frame. As in beamer, plain notes run together as text, each
+starting a new paragraph, and `\note[item]{...}` adds an entry to a numbered
+list printed after them.
+
+Build the deck, then assemble the presenter PDF:
+
+```
+./mknotes example
+```
+
+That reads `example.pdf` and `example.notes` and writes `example-notes.pdf`,
+the double-width version you present from. The slides in `example.pdf` are
+untouched.
+
+If `fonts.tex` is present it is used for the notes pages, so they match the
+deck. `--preamble FILE` names a different fragment.
+
+## Theme
+
+`trantor.sty` is [Marco Pompili's](https://github.com/mrc-pop)
+[Metropolis-like theme](https://github.com/mrc-pop/trantor) for
+ltx-talk.
+
+## Known gaps
+
+Frame titles must be given as `\frametitle{...}` inside the frame, not as the
+argument to `\begin{frame}{...}`. Only the command form is tagged.
+
+ltx-talk tags frame titles at H4, well below sections, and its manual calls the
+relationship between frame titles and sectioning "still very much open".
+
+[Issue #33](https://github.com/josephwright/ltx-talk/issues/33): `\only<1>` and
+`\only<2>` produce output that fails validation. The `\pause` and `<+->` forms
+used here are unaffected.
+
+ltx-talk is version 0.5.x and warns that almost all interfaces may change.
+`talk-notes.sty` reads two internals, `\g__talk_frame_int` and
+`\g__talk_slide_int` — the first thing to check if a release breaks the notes
+build.