Extend README with flashing instructions

Author: krzmaz

README.md

@@ -2,6 +2,16 @@
 This example was described in more detail in a blog post: https://krzmaz.github.io/2022-08-15-creating-a-web-server-on-raspberry-pi-pico-w-using-pico-sdk-and-lwip/  
 In my testing it reached ~20x performance of the MicroPython implementation from point 3.9.1 of https://datasheets.raspberrypi.com/picow/connecting-to-the-internet-with-pico-w.pdf
 
+If you want to test this example with your html files, just add/edit them in the `src/fs` directory and rebuild the project! 
+
+---
+**NOTE**
+
+This project was only tested on Linux and MacOS, I haven't prepared it for Windows.  
+If you need that, feel free to contribute a PR or reach out in the issue section!
+
+---
+
 ### One time setup
 ```bash
 cp cmake/credentials.cmake.example cmake/credentials.cmake
@@ -12,6 +22,9 @@ cp cmake/credentials.cmake.example cmake/credentials.cmake
 ./build.sh
 ```
 
+### Flashing
+The built USB Flashing Format file will be located in `build/src/pico_w_webserver.uf2` - just copy it over to the Pico W to flash it!
+
 ### SSI
 Now the example uses Server Side Includes (SSI) to introduce dynamic data into the HTML responses.  
 As per documentation: