icons: Add README.md

This file intends to capture all knowlege about the purpose and use of
icon files.

Author: gdt

icons/README.md

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+# Unison icons
+
+This directory contains icons for use with unison.
+
+# Known purposes
+
+## Unix Desktop Environments
+
+The square icon (U.svg) could be installed in share/icons/hicolor for
+use by desktop environments.
+
+The
+[icon theme specification](https://specifications.freedesktop.org/icon-theme/latest/)
+requires a raster format, preferring png, and optionally svg.
+The minimal requirement is a 48x48 icon for hicolor.
+
+## Windows
+
+Windows needs a
+[`.ico`](https://en.wikipedia.org/wiki/ICO_(file_format))file.
+Currently one is present in src/win32rc, but there is no information
+about where it came from or how it was created.
+
+The command `icotool` from the package
+[`icoutils`](https://www.nongnu.org/icoutils/) (last release in 2018)
+appears to be able to create ico files and extract images from ico
+files.
+
+According to wikipedia, starting with Windows Vista, `png` is
+acceptable within `ico`.
+
+# Variants
+
+U.svg is a square icon with just a U with arrows.
+
+unison.svg uses the U (apparently from U.svg) and adds "nison" to make
+a word icon.
+
+\todo Consider renaming U to unison and unison to unison-word.  It
+seems likely that the bits currently in U are most useful, and they
+should have a more understandable installed name when installed.
+
+# Build System Issues
+
+Currently, there is no automated or scripted support to convert the
+svg-format files to `png` or `ico`, and the derived formats are
+checked in.  Further, the files within `U.ico` do not appear to all
+have matching `.png` files.