Emphasize `sender`/`psender` usage of `msg.sender` in README

When used in `property` mode with `echidna_x`, `psender` defines the address transactions originate from (from a single address).

When used in `assertion mode`, echidna uses `sender`, in which it originates from 3 addresses.

https://github.com/crytic/echidna/blob/v2.0.0-b2/tests/solidity/basic/default.yaml#L35-L36

This is not explicitly clear unless you look at the `default.yaml`.