Explain the contents of noto-fonts /unhinted

[davelab6]

We offer otf, ttf, variable-ttf and slim-variable-ttf; we should explain what they are and why/when to not/use each

[davelab6]

@marekjez86 mentioned on a call that slim is only 400.700 for Android system UI use only; also, thesee can potentially be mega merged as not all families have 100..900.