Soft vs hard key derivation

[keorn]

This terminology is not common, but used around Substrate:

Two forms of deriving a key, one which allows proof-of-parent (soft), one which does not (hard). Both are allowed. UI/RPC can create derivative keys at will, which yield an address. These addresses can be used to sign with the same security requirements as their root.

[lsaether]

High level information can be added to the keys page on the wiki.

Substrate Dev Hub already has a section on HD derivation that can be used as a touch point.

Bonus points for updating the subkey README.