docs: document the dash analogue of BIP143

[panleone]

Alternative to the old PR #95

This PR describes how the transaction sighash algorithm of BIP143 can be adapted to dash. There are only 2 main differences:

1. This algorithms takes in account dash special tx fields nType and vExtraPayload
2. Algorithm is applied only to transactions with nVersion = 4 and it's deployed with an ehf hard fork.

The example in the test section was created with a draft implementation of the algorithm (see dashpay/dash#5860)

[panleone]

Reviews applied and updated the test to make it consistent with the new approach

[thephez]

Thanks for the work on this! I've added a few suggestions.

[panleone]

Thanks for the work on this! I've added a few suggestions.

thanks for the review, I have applied your suggestions