Add README for FPSViewer tool

Added detailed information about FPSViewer, including features, permissions, requirements, and installation instructions.

Author: binhmod

README.md

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+# FPSViewer
+
+**FPSViewer** is a lightweight Android tool that displays a realtime FPS counter on screen.
+
+The app runs a floating overlay that shows the current frames per second of the active application.
+
+⚡ APK size: **~30 KB**
+
+---
+
+## Features
+
+- 📊 Realtime FPS counter
+- 📱 Floating overlay
+- 🔍 Shows foreground package name
+- ⚡ Very low CPU usage
+- 🪶 Extremely lightweight
+
+---
+
+## Permissions
+
+FPSViewer requires elevated privileges to access system frame statistics.
+
+Supported methods:
+
+- **Shizuku**
+- **Root**
+
+---
+
+## Requirements
+
+- Android 8.0+
+- Overlay permission
+- One of the following:
+  - Shizuku running
+  - Root access
+
+---
+
+## How It Works
+
+FPSViewer reads frame rendering statistics from the Android system and calculates FPS in realtime.  
+The result is displayed as a floating overlay above all apps.
+
+---
+
+## Installation
+
+1. Download the APK from the **Releases** section
+2. Install the APK
+3. Grant **Display over other apps** permission
+4. Start Shizuku or ensure Root access
+5. Launch FPSViewer
+
+---