update README.md with connection pool notes
update README.md with connection pool notes

thomasnield/rxjava-jdbc,tietang/rxjava-jdbc,EmteZogaf/rxjava-jdbc,davidmoten/rxjava-jdbc,EmteZogaf/rxjava-jdbc,AngryX/rxjava-jdbc,davidmoten/rxjava-jdbc,thomasnield/rxjava-jdbc,vtsyryuk/rxjava-jdbc,AngryX/rxjava-jdbc,vtsyryuk/rxjava-jdbc,tietang/rxjava-jdbc