add a note about where all the icons are located

build/shared/lib/icons/README.md

@@ -0,0 +1,99 @@
+This folder contains several icons, so we'll use that as an opportunity to outline where *all* the icons are located in the repo. 
+
+Image2Icon.app on macOS is used to create the `.icns` and `.ico` files from the Illustrator artwork that's been exported to `.png`. It's also used to create `.iconset` folders that contain the icons at several sizes. These are (manually) renamed to the versions seen below.
+
+
+## in this folder
+
+Application icon for the PDE, used for the dock and windows:
+
+    build/shared/lib/icons/app-16.png
+    build/shared/lib/icons/app-32.png
+    build/shared/lib/icons/app-48.png
+    build/shared/lib/icons/app-64.png
+    build/shared/lib/icons/app-128.png
+    build/shared/lib/icons/app-256.png
+    build/shared/lib/icons/app-512.png
+    build/shared/lib/icons/app-1024.png
+
+The 48x48 version is created from the 64x64 image using ImageMagick:
+
+    convert -resize 48x app-64.png app-48.png
+
+The document icon for `.pde` files. (Prior to 4.3, the `pde-NN.png` files were the app icon, causing some confusion.) These images are only used on Linux:
+
+    build/shared/lib/icons/pde-16.png
+    build/shared/lib/icons/pde-32.png
+    build/shared/lib/icons/pde-48.png
+    build/shared/lib/icons/pde-64.png
+    build/shared/lib/icons/pde-128.png
+    build/shared/lib/icons/pde-256.png
+    build/shared/lib/icons/pde-512.png
+    build/shared/lib/icons/pde-1024.png
+
+And to create that 48x48 version:
+
+    convert -resize 48x pde-64.png pde-48.png
+
+Finally, there's the Foundation logo, which is used by the Contributions Manager:
+
+    build/shared/lib/icons/foundation-16.png
+    build/shared/lib/icons/foundation-32.png
+    build/shared/lib/icons/foundation-64.png
+
+
+## inside `core`
+
+The exported application icon, used to set the dock icon when running a sketch, or in an exported application:
+
+    core/src/icon/icon-16.png
+    core/src/icon/icon-32.png
+    core/src/icon/icon-48.png
+    core/src/icon/icon-64.png
+    core/src/icon/icon-128.png
+    core/src/icon/icon-256.png
+    core/src/icon/icon-512.png
+    core/src/icon/icon-1024.png
+
+And of course:
+
+    convert -resize 48x icon-64.png icon-48.png
+
+
+## exported applications
+
+Exported application icon for macOS:
+
+    java/application/application.icns
+
+…and for Windows:
+
+    java/application/application.ico
+
+There's none in use for Linux, since we don't export `.desktop` files with applications; though maybe we should do that in the future.
+
+
+## macOS build
+
+Application icon:
+
+    build/macos/processing.icns
+
+Document icons, named by extension:
+
+    build/macos/pde.icns
+    build/macos/pdex.icns
+    build/macos/pdez.icns
+
+
+## Windows build
+
+Application icon:
+
+    build/windows/processing.ico
+
+Document icons, named by extension:
+
+    build/windows/pdex.ico
+    build/windows/pdez.ico
+    build/windows/pde.ico