[Enhancement] Provide variable for release notes

[brutaldev]

Can use Ketarin to extract the release notes into a variable which needs to be passed in as a replacement parameter to. The variable can replace a placeholder in <releaseNotes> of the .nuspec file.

For some users it's important to know the changes before applying an update and this will help get rid of the generic "This is an automatically updated package" from the template. If no variable is provided then the default template message can be used as a fallback.