Update prebuilds docs

[vweevers]

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When installing leveldown, prebuild-install will install prebuilt binaries from GitHub if they exist and fallback to a compile step if they don't. In that case you'll need a valid node-gyp installation.

If you don't want to use the prebuilt binary for the platform you are installing on, specify the --build-from-source flag when you install. If you are working on leveldown itself and want to re-compile the C++ code it's enough to do npm install.