Document changed signature for sign_ps256

[crandmck]

From sdk 0.4 to 0.6, there was a change to c2pa.sign_ps256 which causes this error if not addressed:

"bad parameter: Other TypeError('ECPrivateKey.sign() takes 2 positional arguments but 3 were given')"

Parameters to that function changed. It used to take a file path and now it expects the PEM cert string. Since they are both strings, it's confusing. It's more flexible not depending on file I/O.

From Discord.