Add a README to the docs/ dir.

docs/README.md

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+# Documentation for Thun
+
+This directory contains the docs for Thun.  It's kind of a mess right now
+(Aug 2023) but I'm working on it.  Originally the docs took the form of a
+static website created with Sphinx (TODO: link to Sphinx project) and
+uploaded to OSDN (which provides a static site hosting service as part of
+your project there.)  This was great, but recently I've been having
+little troubles with reaching OSDN.  It seems weird to have issues in
+2023 (Japan's not that far away?) but there it is.
+
+Now I'm trying a simple collection of Markdown files which get converted
+to HTML by a simple Python script (the reference pages still use Pandoc.)
+I want to have a few pages covering Thun project itself, but most of the
+docs should be the notebooks and the function reference.  (Some of the
+implementation languages have automatic doc generators and it might be
+nice to imclude those in the docs, but getting them all to look like each
+other might be bleah.)
+
+## Directory
+
+- `build_index.py` a crude script to convert Markdown to HTML
+- `html/` output of the HTML creation process.  Serve the contents of
+  this directy.
+- `_joy_interpreter_flowchart_url.txt`  This is the URL for the SVG
+  flowchart image generator service. 
+- `Makefile`  uses `build_index.py` to convert `source/` to `html/`.
+- `notebooks/` The original Jupyter notebooks (`*.ipynb` files).  I have
+  a simple Joy kernel but I no longer know how to install and use it, so
+  I'm converting these to Markdown files in the `source/notebooks`
+  directory.
+- `OSDN-Makefile` The old make file for OSDN, took care of uploading.
+- `reference/` The function reference docs.  Some of these are out of
+  date (e,g, some of the names of functions have changed.)  There's a
+  script to convert the separate per-function Markdown files into a
+  single-page HTML file.
+- `source/`  The new source directory for generating the static site (in
+  the `html/` directory.)
+