It's pretty good, and was needed. It is about time someone ends the "NES ROMs can't be expanded" fallacy.
I think you should add the following :
1) In some cases, it's really impossible to exand the ROM (for example MMC1 can't support more than 512kb PRG-ROM, or 256kb PRG if CHR-ROM is used).
2) You say most of the time CHR-ROM is used, but you could also mention than in ~40% of the time games use CHR-RAM instead, and thus it's normal no CHR-ROM data is stored in the iNES dump. (just so that they don't complain "hey, where is the CHR-ROM ?")
3) You should definitely mention that the size of the PRG-ROM *must* be a power of two, so if you expand it, you must double it's size. I think many people will try to be smart and expand just one bank or something.
On an unrelated note, the power of data compression should not be forgotten. I should update my CompressTools so they don't require Java anymore, and pehaps add better doccumentation to them, so people will be more likely to use them.