docs: update README

Author: redchenjs

README.md

@@ -1,8 +1,17 @@
-Bluetooth Speaker
-=================
+ESP32 Bluetooth Speaker
+=======================
 
 Bluetooth Speaker based on ESP32 chip with optional visual effects output.
 
+## Main Features
+* A2DP Music Streaming
+* I2S Output (External DAC needed)
+* VFX Output (Audio FFT / Rainbow / Stars / ...)
+* BLE Control Interface (for VFX Output)
+* Audio Prompt (Connected / Disconnected / Resume / Sleep)
+* Sleep/Resume (External Key needed)
+* OTA Firmware Update (via SPP Profile)
+
 ## Music VFX on ST7735 80x160 LCD Panel (Realtime FFT)
 <img src="docs/lcd.png">