Build instructions in README.md

[Ellis Whitehead]

Could you provide the basic instructions in README.md to build the ISO?

[Stephan Henningsen]

Hi Ellis!

My recommendation is always to use the official ISO like everyone else. That way your experiments are performed on a common ground, and any bugs or improvements you report will benefit everyone in the Lightwhale community.

Lightwhale is in fact a project-specific customization for the Buildroot project, which already does a great job documenting the build process. In short, get the correct buildroot-verison to build Lightwhale, point the $BR2_EXTERNAL to the custom directory of the Lightwhale source tree, and follow the build instructions: https://buildroot.org/downloads/manual/manual.html#outside-br-custom

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