Add logo attribution for Android, HTML5 and Linux platform icons

- Tweak the Android platform logo to remove the Android wordmark,
  as it can't be used without explicit permission.

platform/android/README.md

@@ -0,0 +1,14 @@
+# Android platform port
+
+This folder contains the Java and C++ (JNI) code for the Android platform port,
+using [Gradle](https://gradle.org/) as a build system.
+
+## Artwork license
+
+[`logo.png`](logo.png) and [`run_icon.png`](run_icon.png) are licensed under
+[Creative Commons Attribution 3.0 Unported](https://developer.android.com/distribute/marketing-tools/brand-guidelines#android_robot)
+per the Android logo usage guidelines:
+
+> The Android robot is reproduced or modified from work created and shared by
+> Google and used according to terms described in the Creative Commons 3.0
+> Attribution License.

platform/javascript/README.md

@@ -0,0 +1,15 @@
+# HTML5 platform port
+
+This folder contains the C++ and JavaScript code for the HTML5/WebAssembly platform port,
+compiled using [Emscripten](https://emscripten.org/).
+
+It also contains a ESLint linting setup (see [`package.json`](package.json)).
+
+See also [`misc/dist/html`](/misc/dist/html) folder for files used by this platform
+such as the HTML5 shell.
+
+## Artwork license
+
+[`logo.png`](logo.png) and [`run_icon.png`](run_icon.png) are licensed under
+[Creative Commons Attribution 3.0 Unported](https://www.w3.org/html/logo/faq.html#how-licenced)
+per the HTML5 logo usage guidelines.

platform/linuxbsd/README.md

@@ -0,0 +1,11 @@
+# Linux/*BSD platform port
+
+This folder contains the C++ code for the Linux/*BSD platform port.
+
+## Artwork license
+
+[`logo.png`](logo.png) is derived from the [Linux logo](https://isc.tamu.edu/~lewing/linux/):
+
+> Permission to use and/or modify this image is granted provided you acknowledge me
+  <lewing@isc.tamu.edu> and [The GIMP](https://isc.tamu.edu/~lewing/gimp/)
+  if someone asks.