Add documentation to explain how to terminate the `bn128` curve

[cedoor]

Related: #318

Example: https://github.com/0xPolygonID/js-sdk/blob/main/src/proof/provers/prover.ts#L91-L92

[0xshikhar]

Hey @cedoor , would like to work on this task if anyone is not working on it.
Any additional context would be appreciated. Thanks!