Add README and license

Author: morozov

LICENSE

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+River Raid is Copyright (c) 1982/1984 Activision. This project is an
+independent remake and is not affiliated with or endorsed by Activision.
+
+MIT License
+
+Copyright (c) 2026 Sergei Morozov
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.

README.md

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+# River Raid (Ebiten)
+
+A remake of the ZX Spectrum 48K game **River Raid** (1984, Activision) in Go using
+the [Ebiten](https://ebitengine.org/) game engine. The game logic was reverse-engineered from the original via
+an [annotated disassembly](https://river-raid.github.io/river-raid-disasm/)
+([source](https://github.com/river-raid/river-raid-disasm)).
+
+<p align="center"><img src="docs/gameplay.png" alt="Gameplay screenshot"></p>
+
+## Play Online
+
+A WebAssembly build is available at [river-raid.github.io](https://river-raid.github.io/).
+
+## Future: Modernized Remake
+
+The faithful reimplementation is a foundation for a potential modernized remake. The mechanics would be preserved
+exactly as reverse-engineered, with the presentation brought up to date:
+
+- **Resolution** — render at native target resolution rather than scaling up the 256×192 ZX Spectrum screen
+- **Frame rate** — smooth gameplay to 60 FPS; the original's 12 FPS is a result of suboptimal implementation, not a
+  design choice
+- **Sprites** — replace the monochrome pixel bitmaps with full-color, higher-resolution artwork
+- **Audio** — replace the 1-bit beeper with sampled sound effects and music
+- **Touch controls** — make the game playable on mobile devices