add SourceLink.MilestoneReleaseNotes

[ctaggart]

No description provided.

[ctaggart]

Test successfully with:

dotnet tool install --global SourceLink.MilestoneReleaseNotes --version 3.0.0-build.778 --add-source https://ci.appveyor.com/nuget/sourcelink

I tested with environment variables:

$env:RELEASENOTES_GITHUBTOKEN = "mytoken"
$env:RELEASENOTES_GITHUBOWNER = "ctaggart"
$env:RELEASENOTES_GITHUBREPOSITORY = "sourcelink"
$env:RELEASENOTES_MILESTONE = "3.0.0"
milestonereleasenotes

I also ran it by specifying args:

milestonereleasenotes githubowner=ctaggart githubrepository=sourcelink milestone=3.0.0

dotnet tool uninstall SourceLink.MilestoneReleaseNotes -g