Doc: how to use with Platform Identity Keys (DIP 13)

[coolaj86]

m / purpose' / coin_type' / feature' / sub feature' /

// m/9'/5'/5'/0'/0'/0'/0'
purpose = 9'
coin_type = 5'
feature = 5' // dash identity DIP13
sub_feature = 0' // Identity Authentication keys
key_type = 0' // ECDSA 1' BLS
indentity_index = n'
key_index = n' // multiple keys per identity

// m/9'/5'/5'/0'/0'/0'/0'
purpose = 9'
coin_type = 5'
feature = 5'
sub_feature = 1' // Identity Registration funding
identity_index = n

As there can only be one registration funding per identity, there is no point to derive a level deeper.

// Identity Top Up Funding keys
// m / purpose' / coin_type' / feature' / sub feature' / funding index
purpose = 9'
coin_type = 5'
feature = 5'
sub_feature = 2' // top-up
identity_index = n