Document the use of shadow registers like hLCDC and hBGP.

[nitro2k01]

I've seen two people on the Discord confused about the use of shadow registers just in the last week. The first person had the problem that the LCD was immediately disabled after enabling it. The issue turned out to be that the shadow register handler wrote the value of hLCDC (00) on the next VBlank. The second user noted that changes to rBGP would only work, as they put it, if you wrote to the register in an infinite loop.

I would suggest documenting how the shadow registers work and the caveats around using the code included in the gb-starter-kit, so there's a single place to point to if the question arises again. This should probably be mentioned briefly, maybe with an appropriate link, in intro.asm which is likely where a new user will look when using the project for the first time.

I've looked both in the GB ASM style guide and GB ASM tutorial in case anything about it was mentioned there, but nothing as far as I could see.