Please clarify in the readme that not all GB 18030 characters in the SC/CN version of the fonts follow the PRC glyph conventions

I know this issue has been reported several times before, but this pertains to the readme, although I think it will only be updated when a new version of Source Han Sans/Serif is released.

Due to limited glyph space, some Chinese characters now show JP forms or [TW/HK forms](https://github.com/adobe-fonts/source-han-sans/issues/313) as the only glyph shape for CN. Granted those characters are occasionally or rarely encountered in a Chinese context. I have since [listed](https://github.com/adobe-fonts/source-han-sans/issues/432) down every single GB18030 character showing JP forms and my suggestions on whether to restore them.

This above graphic is just an example of some hanzi following non-standard forms and using STHeiti (a Mac font) as a reference for PRC glyph conventions (although yes, the reference may not be entirely accurate).

In the _Glyph Set & Region-specific Subs_ section of the readme, it does state that for Traditional Chinese, "the glyphs may or may not adhere to the Taiwan MOE glyph standard". Adobe can try to get as much GB 18030 characters to follow CN conventions as possible with a major redesign of Sans (and merging unnecessary regional components), but because of the aforementioned 65K glyph limit, it is likely they won't be able to get all of them to do so. Therefore and regardless, the readme should be updated to state that "some glyphs may not adhere to the PRC glyph conventions due to limited glyph space" or something similar for the _Simplified Chinese_ row.

This issue also applies to Serif, although the good thing is there are less CN glyphs removed than in Sans.

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**EDIT:** The known issues section in Sans 2.004 does not mention anything about this issue.

Same for Serif 2.002, and I filed this very issue months before its release.

I have also edited the issue title several times for clarity.

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**EDIT 2:** I moved the list of GB standards here because I have deleted some inappropriate comments, for organizational purposes.

Here is the list of GB standards according to Unicode which I believe are basic standards for characters to follow the PRC glyph conventions:
> - G0 GB/T 2312-1980 (formerly GB 2312-80)
> - G1 GB/T 12345-1990 (formerly GB/T 12345-90)
> - G7 General Purpose Hanzi List for Modern Chinese Language, and General List of Simplified Hanzi
> - G8 GB/T 8565.2-1988 (formerly GB 8565.2-88)

Reference: https://www.unicode.org/reports/tr38/tr38-33.html

If there is any GB list in the reference link that I missed out on (or if I put a standard that is not necessary), let me know.