Add readme

Author: burner1024

README.md

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+# Fallout 2 damage calculator
+
+[Calculate](https://fodc.bgforge.net)
+
+[Discuss](https://forums.bgforge.net/viewtopic.php?p=2567)
+
+### Build
+
+```bash
+pnpm install
+pnpm start
+```
+
+#### Update data
+
+Update `data/mods/*.csv`, then
+
+```bash
+./data/update.sh
+```