Add Linux keyring backend requirement docs to README & manual install docs

[cap10morgan]

On Linux the keyring backends aren't always pre-installed and there are a few different options supported. We should document the options and prerequisites there.

[Delivator]

What options do I have?

[cap10morgan]

What options do I have?

Currently any of these should work (though I don't think we've tested them all yet):

dgit/keyring/keyring.go

Lines 30 to 36 in 9d452cf

	var KeyringPrettyNames = map[string]string{
	"*keyring.keychain": "macOS Keychain Access",
	"*keyring.kwalletKeyring": "KWallet (KDE Wallet Manager)",
	"*keyring.windowsKeyring": "Windows Credential Manager",
	"*keyring.secretsKeyring": "libsecret",
	"*keyring.passKeyring": "pass",
	}

[xloem]

What is "pass"? Is there a path for headless Linux atm?