Add project logo and use it for the app icon

- assets/logo.png (transparent) and assets/logo-256.png for the site
  and README
- tools/make_icon.m builds the full-bleed iOS icon from the logo

README.md

@@ -1,4 +1,6 @@
-# Factorio on iPhone
+<p align="center"><img src="assets/logo-256.png" width="160" alt="Factorio for iPhone logo"></p>
+
+<h1 align="center">Factorio on iPhone</h1>
 
 Play your own copy of **Factorio** on your iPhone. You build the app on your Mac from the
 Factorio you already own on Steam, and install it on your phone with a cable.
@@ -130,6 +132,9 @@ git pull
 - [docs/research/](docs/research/static-translation.md): earlier research (ARM64 → WASM
   translation).
 
+**Logo:** `assets/logo.png` (transparent, 683 px) and `assets/logo-256.png` are the project's own
+artwork, also used for the app icon. They are not Factorio's official logo.
+
 **Third-party software:** [Wuffs](https://github.com/google/wuffs) (Apache-2.0) is downloaded
 during the build to decode PNG images. Factorio itself uses [SDL](https://github.com/libsdl-org/SDL)
 (zlib). Factorio is a trademark of Wube Software Ltd.; all game code and assets belong to them.