CFF metrics-related checks

Moved from top-level readme to get a better chance of being looked at.

```
Incomplete CFF checks:

val_CFF.cs   I.CFF_I_NotValidated
val_head.cs: I._TEST_I_NotForCFF head_MinMaxValues
val_hhea.cs: I._TEST_I_NotForCFF hhea_MinMax
val_OS2.cs:  I._TEST_I_NotForCFF OS/2_xAvgCharWidth
```