documented how to create hybrid ROM

doc/Hybrid-build.md

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+The C64 ROM is typically distributed in 2 files 8 KB each, one called *kernal*, one *basic*. This might be misleading, as the C64 Kernal takes about 6.5 KB, and the BASIC about 9.5 KB - some parts of BASIC is therefore placed in the *kernal* file. This is the main reason you can't easily use the Open ROMs Kernal file with original C64 BASIC.
+
+But it is still possible to combine them:
+* copy the *kernal* file to the *open-roms* main directory
+* from the Linux command line type `make test_hybrid` - this will crate the hybrid Kernal file, *open-roms/bin/kernal_hybrid.rom*, and launch VICE emulator with it
+
+Sorry, no Windows support (but it should work under *Windows Subsystem for Linux* if GCC and libpng devel packages are installed), and no macOS support (I don't have a machine to test).
+
+Please note, that it is illegal to distribute such a hybrid ROM:
+
+* it contains proprietary Commodore/Microsoft code
+* it contains LGPL 3 licensed Open ROMs code combined with code not available under compatible license