Use system keyring for passwords when available

This change introduces the use of system keyring to store repository
passwords when one is available. Passwords are added to and removed from
the keyring using the config command.

Resolves: python-poetry#210

docs/docs/repositories.md

@@ -49,6 +49,10 @@ If you do not specify the password you will be prompted to write it.
 You can also specify the username and password when using the `publish` command
 with the `--username` and `--password` options.
 
+If a system keyring is available and supported, the password is stored to and retrieved from the keyring. In the above example, the credential will be stored using the name `poetry-repository-pypi`. If access to keyring fails or is unsupported, this will fall back to writing the password to the `auth.toml` file along with the username. 
+
+Keyring support is enabled using the [keyring library](https://pypi.org/project/keyring/). For more information on supported backends refer to the [library documentation](https://keyring.readthedocs.io/en/latest/?badge=latest).
+
 ### Install dependencies from a private repository
 
 Now that you can publish to your private repository, you need to be able to