added pdfs to gitignore
added pdfs to gitignore

ChinaQuants/IRkernel,ibm-et/IRkernel,mikecroucher/IRkernel,Phobia0ptik/IRkernel,chendaniely/IRkernel,rgbkrk/IRkernel,gdtm86/IRkernel,elaOnMars/IRkernel,ibm-et/IRkernel,JanSchulz/IRkernel