Add third-party changes that impact recipe results to changelog

[bouweandela]

When testing ESMValTool recipes for a release, we may find differences in the results w.r.t. the previous release because of changes in third party packages that we depend on. A recent example was the change of the default font in the R ploting library (ref #2704 (comment)). It would be good to summarize these in a new section of the changelog when making a new release. This will save users of the ESMValTool time when they are trying to figure out why their recipes now produce different results.