Add XML doc sample code for Add aliases

[devlead]

For better documentation on cakebuild.net add example code to GitAdd, examples/usage can probably be found in the integration tests.

Example how to XML doc see example tag below

        /// <summary>
        /// Retrieves the value of the environment variable or <c>null</c> if the environment variable does not exist.
        /// </summary>
        /// <example>
        /// <code>
        /// Information(EnvironmentVariable("HOME") ?? "Unknown location");
        /// </code>
        /// </example>
        /// <param name="context">The context.</param>
        /// <param name="variable">The environment variable.</param>
        /// <returns>The environment variable or <c>null</c> if the environment variable does not exist.</returns>