Update README

[mbirth]

Please update the README instructions for newer Keychain versions.

There are now the following input fields:

• Name: "GnuPG"
• Kind: "application password"
• Account: <Keygrip>
• Where: "GnuPG"
• Comments: (empty)
• Password: <Password>

However, it seems it's now looking for a completely different entry in Keychain because using pinentry-mac directly works fine for me (finds above entry), using pinentry-touchid doesn't for some reason.

P.S.: You might also want to distinguish your project from the other pinentry-touchid.