Suggest adding java requirement of PDFMerger node module to install documentation

[lasairport]

Discovered this when people were reporting errors generating PDFs using the export-service. I found errors related to this in the logs. Installed java, added path in my startup (I am using PM2, so had to add in my config - dump.pm2) and things started working.

[ZornitsaPesheva]

Hi lasairport,

Thank you for reporting this. We will have it in mind.