Add reference to third party rollbar starter

[snicoll]

There was a request to add this feature in Spring boot in #14248 which was declined and it's now available as a third party starter that we should add to our list.

[kuldeepsidhu88]

@snicoll Can I pick this one? Is there any other change required apart from the documentation for this?

[snicoll]

@kuldeepsidhu88 thanks for the offer but I've assigned myself as I've already the changes locally. You can look at idea for contribution issues.