Add instruction for building traditionally on UNIX system

[timkenhan]

No description provided.

[akira25]

I like this one. Should speed up development-cycles a lot by shorter build times.

[timkenhan]

@akira25 Glad you are liking it!

Personally, I'm quite bothered the primary build instruction prompts us to use the Github codespace. Due to the large overlap between the tinkerer community and the open-source community, I think it would be more appropriate to feature a more primitive instruction like the one I provided. Meanwhile, the less tech-savvy crowd would be using the web-based flasher provided from the release page, just like they always have been.

I also think that Docker might be overkill for the job, but I understand that not everyone uses the same exact system configurations and Docker would help making it all more uniform. Nevertheless, it puzzles me that the README attributes testing the build on Ubuntu ("Compiler" section) while the Docker uses Arch Linux.

@egzumer I hope you kindly consider my reasoning above. I am open for feedback, especially for getting the build instruction on par with the Docker build output.