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

[will-v-pi]

Add note about pre-built picotool binaries in the pico-sdk-tools repository

[lurch]

"If you don't need..." sounds a bit strange - why would anybody need to build picotool themselves? 😉

Perhaps this could be shortened to something like "Pre-built binaries of picotool for Windows, macOS, and Linux can be downloaded from the pico-sdk-tools repository. But if you want to build it yourself from source..."

Hmm, it's obviously a slightly larger change, but given that we are now offering pre-built binaries, perhaps the "Building" section should come after the "Overview" section? Or perhaps be moved to an entirely separate BUILDING.md file?

[will-v-pi]

I think that keeping the Building and Usage by the SDK sections at the top is useful, as the Overview is very long and therefore people might miss the build instructions, which would probably create more issues.

"If you don't need..." sounds a bit strange - why would anybody need to build picotool themselves? 😉

You might not trust binaries you've downloaded from the internet, or you might want to install picotool system-wide - both of these would require building it yourself, although maybe "If you don't want..." would be better phrasing? I think it's still the main use case for people who arrive at this repository, and building it from source should work fine across all platforms - it works fine on Windows for me

[lurch]

Fair enough. But perhaps it's worth having a brief "Introduction" or "Description" at the top of README.md, rather than diving straight into "Building" on the very first line? 🤷