Document how to add reference_inputs when using TransactionBuilder

[peterVG]

It's not obvious how to set values for the reference_inputs property and add it to a TransactionBody, as per the API here:
https://pycardano.readthedocs.io/en/latest/api/pycardano.transaction.html#pycardano.transaction.TransactionBody

I can add another TransactionBuilder property like ttl as part of the object instantiation, e.g.
builder = TransactionBuilder(context=context, ttl=2024)

But trying to do the same for the reference_inputs property results in:

builder = TransactionBuilder(context=context, reference_inputs=["testing1", "testing2"])
>>>TypeError: TransactionBuilder.__init__() got an unexpected keyword argument 'reference_inputs'

Trying to add it to the object after it's instantiated also fails to build a proper transaction:

builder = TransactionBuilder(context)`
builder.__set__(self, reference_inputs, {"test1", "test2"})
>>>AttributeError: 'TransactionBuilder' object has no attribute '__set__'. Did you mean: '__eq__'?

This code here appears to be adding reference_inputs to the utxo.output.script but, again, not sure where to initially set that reference input value.

pycardano/pycardano/txbuilder.py

Lines 224 to 227 in c5430c0

	if utxo.output.script:
	self._inputs_to_scripts[utxo] = utxo.output.script
	self.reference_inputs.add(utxo.input)
	self._reference_scripts.append(utxo.output.script)

[cffls]

Because reference inputs under txbuilder is an empty set by default, you need to add reference inputs like this:

builder.reference_input.add(my_input)

or this:

builder.reference_input = {my_input1, my_input2}

You can also find an example of using reference script in this test: https://github.com/Python-Cardano/pycardano/blob/main/integration-test/test/test_plutus.py#L239

[peterVG]

OMG. I tried everything but that :-)

Thanks very much @cffls I can confirm this works. Just one correction, the property name should be plural e.g. builder.reference_inputs not builder.reference_input

Will return favour with a documentation PR that you may or may not want to add.

[peterVG]

See PR #119