request to add documentation for RSA subkey size

[leifliddy]

So the documentation currently shows the following for add_subkey:

The add_subkey method has some additional keyword arguments:

algorithm (defaulting to rsa)
usage (defaulting to encrypt)
expire (defaulting to -)

But it doesn't show how to request an RSA key of a specific size.
The default RSA key size is dependent upon the version of gpg installed.
So it's useful to know how to specify the key size.

The RSA key size can easily be defined via the following notation:

gpg.add_subkey(..., algorithm='rsa2048')
gpg.add_subkey(..., algorithm='rsa3072')
gpg.add_subkey(..., algorithm='rsa4096')

Could you please include that in your documentation?

[vsajip]

Sure, I'll do that.