update c readme (Picovoice#483)

Author: ksyeo1010

demo/c/README.md

@@ -14,12 +14,18 @@ Picovoice requires a valid Picovoice `AccessKey` at initialization. `AccessKey`
 You can get your `AccessKey` for free. Make sure to keep your `AccessKey` secret.
 Signup or Login to [Picovoice Console](https://console.picovoice.ai/) to get your `AccessKey`.
 
-## Build
+## Build Linux/MacOS
 
 ```console
 cmake -S demo/c/. -B demo/c/build && cmake --build demo/c/build --target picovoice_demo_mic
 ```
 
+## Build Windows
+
+```console
+cmake -S demo/c/. -B demo/c/build -G "MinGW Makefiles" && cmake --build demo/c/build --target picovoice_demo_mic
+```
+
 ## Run
 
 ### Usage