Add hint on pull requests to CONTRIBUTING.md
Add hint on pull requests to CONTRIBUTING.md

Braunch/jabref,grimes2/jabref,jhshinn/jabref,Siedlerchr/jabref,tobiasdiez/jabref,tschechlovdev/jabref,mredaelli/jabref,motokito/jabref,sauliusg/jabref,jhshinn/jabref,ayanai1/jabref,obraliar/jabref,Siedlerchr/jabref,Siedlerchr/jabref,zellerdev/jabref,Mr-DLib/jabref,mairdl/jabref,bartsch-dev/jabref,mredaelli/jabref,sauliusg/jabref,Mr-DLib/jabref,zellerdev/jabref,obraliar/jabref,tschechlovdev/jabref,oscargus/jabref,tobiasdiez/jabref,sauliusg/jabref,tschechlovdev/jabref,obraliar/jabref,Mr-DLib/jabref,shitikanth/jabref,obraliar/jabref,mairdl/jabref,obraliar/jabref,zellerdev/jabref,shitikanth/jabref,JabRef/jabref,ayanai1/jabref,zellerdev/jabref,motokito/jabref,grimes2/jabref,bartsch-dev/jabref,motokito/jabref,Siedlerchr/jabref,oscargus/jabref,bartsch-dev/jabref,mairdl/jabref,ayanai1/jabref,ayanai1/jabref,shitikanth/jabref,JabRef/jabref,shitikanth/jabref,Braunch/jabref,jhshinn/jabref,oscargus/jabref,Braunch/jabref,grimes2/jabref,bartsch-dev/jabref,ayanai1/jabref,motokito/jabref,mredaelli/jabref,shitikanth/jabref,mredaelli/jabref,Mr-DLib/jabref,grimes2/jabref,Mr-DLib/jabref,oscargus/jabref,tobiasdiez/jabref,tobiasdiez/jabref,grimes2/jabref,bartsch-dev/jabref,jhshinn/jabref,Braunch/jabref,jhshinn/jabref,tschechlovdev/jabref,sauliusg/jabref,tschechlovdev/jabref,oscargus/jabref,zellerdev/jabref,mredaelli/jabref,mairdl/jabref,mairdl/jabref,JabRef/jabref,motokito/jabref,Braunch/jabref,JabRef/jabref