Add note about pico-sdk-tools to README.md

Author: will-v-pi

README.md

@@ -1,5 +1,7 @@
 ## Building
 
+> If you don't need to build picotool yourself, you can find pre-built binaries for Windows, macOS, and Linux in the [pico-sdk-tools](https://github.com/raspberrypi/pico-sdk-tools/releases) repository
+
 You need to set PICO_SDK_PATH in the environment, or pass it to cmake with `-DPICO_SDK_PATH=/path/to/pico-sdk`. To use features such as signing or hashing, you will need to make sure the mbedtls submodule in the SDK is checked out - this can be done by running this from your SDK directory.
 
 ```console