Installation: Just go to Peppermint->Extension Manager and in the Plugins tab click on the "+" button. Then selected the "Subtitles Synchronizer.ppPlugin" file. That's it!
Usage: Open any .srt file and while on the file, go to the Tools menu and click on "Subtitle Synchronizer". An input dialog will popup. Just add the number of milliseconds you want (either positive, in case you want to add a "delay", or negative) and click "OK". In a couple of... milliseconds, the adjusted and synchronized subtitles will appear next to your original subtitles. :)