Add all logo variants and sizes

- generated source images (white background, rounded tile)
- transparent logo in 512/128/64/32 px
- app icon 1024 px plus 180/32/16 px
- assets/logo/README.md describing each file

assets/logo/README.md

@@ -0,0 +1,17 @@
+# Logo files
+
+Project artwork for the website, the repository and the iPhone app. Original artwork for this fan
+project; not Factorio's official logo.
+
+| File | Use |
+| --- | --- |
+| `../logo.png` | Main logo, transparent background, 683×683 (master for everything below) |
+| `../logo-256.png` | README header |
+| `logo-512.png`, `logo-128.png`, `logo-64.png`, `logo-32.png` | Transparent logo in common sizes (website, favicon) |
+| `app-icon-1024.png` | iPhone app icon: full square, opaque, no rounded corners (iOS rounds it) |
+| `app-icon-180.png`, `app-icon-32.png`, `app-icon-16.png` | Same icon, smaller (Apple touch icon, favicons) |
+| `source-white-background.jpg` | Generated original on white, from which `logo.png` was cut out |
+| `source-rounded-tile.jpg` | First generated version, with a rounded dark tile |
+| `preview-dark-and-magenta.png` | Check of the transparent logo on dark and magenta backgrounds |
+
+The app build regenerates the iPhone icon from `../logo.png` with `tools/make_icon.m`.