Add documentation XML to NuGet pacakge

## Expected Behavior

The addin contains XML comments for addins, but doesn't ship with the documentation XML file. Therefore comments can't be shown on Cake website (e.g. https://cakebuild.net/extensions/cake-travisci/)).

## Current Behavior

No comments /examples are shown for the aliases provided by this addin.

## Possible Solution

Include the documentation XML in the NuGet package

## Steps to Reproduce (for bugs)

1. Go to addin detail page at https://cakebuild.net/extensions/cake-travisci/

## Context

N/A

## Your Environment

* Addin version used: 0.4.1
* Cake Version used: N/A
* Operating System: N/A

Inspect the repository's NuGet packaging configuration and the project settings that produce the addin assembly. Verify that the documentation XML is included in the generated package and that the Cake addin page can display comments and examples for the provided aliases.

Written by the indexing model from the issue text.