add linux build instructions to README.md

[aszarth]

Added instructions on how to build on Linux.
Also included information about sub module folders for users who don't use the --recursive option, so they can manually check and download them.

[CLAassistant]

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[AmyrAhmady]

Also one more question, why are you encouraging to use docker manually and directly when there's a build.sh script file for anyone to use easily, without requiring them to switch between different ubuntu version folders, also able to set some configurations