doc: add gitter chat link to readme
doc: add gitter chat link to readme

Closes #470

golovnin/pgjdbc,sehrope/pgjdbc,pgjdbc/pgjdbc,golovnin/pgjdbc,jorsol/pgjdbc,AlexElin/pgjdbc,lordnelson/pgjdbc,zapov/pgjdbc,jamesthomp/pgjdbc,whitingjr/pgjdbc,lordnelson/pgjdbc,jamesthomp/pgjdbc,AlexElin/pgjdbc,pgjdbc/pgjdbc,panchenko/pgjdbc,whitingjr/pgjdbc,pgjdbc/pgjdbc,davecramer/pgjdbc,zapov/pgjdbc,zemian/pgjdbc,lordnelson/pgjdbc,sehrope/pgjdbc,jamesthomp/pgjdbc,jkutner/pgjdbc,davecramer/pgjdbc,panchenko/pgjdbc,davecramer/pgjdbc,marschall/pgjdbc,whitingjr/pgjdbc,sehrope/pgjdbc,jorsol/pgjdbc,pgjdbc/pgjdbc,zemian/pgjdbc,AlexElin/pgjdbc,Gordiychuk/pgjdbc,davecramer/pgjdbc,zapov/pgjdbc,golovnin/pgjdbc,marschall/pgjdbc,zemian/pgjdbc,marschall/pgjdbc,jkutner/pgjdbc,marschall/pgjdbc,jorsol/pgjdbc,Gordiychuk/pgjdbc,sehrope/pgjdbc,panchenko/pgjdbc,jkutner/pgjdbc,Gordiychuk/pgjdbc,jorsol/pgjdbc