docs/config: document SMTP settings
docs/config: document SMTP settings

Related to #200.

mderaldo/tsuru,kgrvamsi/tsuru,RichardKnop/tsuru,guilhermebr/tsuru,tuxmonteiro/tsuru,tsuru/tsuru,Endika/tsuru,tsuru/tsuru,kgrvamsi/tsuru,cezarsa/tsuru,mderaldo/tsuru,gciotta/tsuru,guilhermebr/tsuru,cezarsa/tsuru,hcxiong/tsuru,jblancett/tsuru,gwmoura/tsuru,tuxmonteiro/tsuru,cgvarela/tsuru,firebitsbr/tsuru,ggarnier/tsuru,hugoantunes/tsuru,cgvarela/tsuru,hcxiong/tsuru,hugoantunes/tsuru,gwmoura/tsuru,Endika/tsuru,guilhermebr/tsuru,ggarnier/tsuru,ArxdSilva/tsuru,firebitsbr/tsuru,kgrvamsi/tsuru,RichardKnop/tsuru,gciotta/tsuru,mderaldo/tsuru,tuxmonteiro/tsuru,hcxiong/tsuru,ArxdSilva/tsuru,guilhermebr/tsuru,ArxdSilva/tsuru,ggarnier/tsuru,ggarnier/tsuru,ArxdSilva/tsuru,tsuru/tsuru,tsuru/tsuru,Endika/tsuru,firebitsbr/tsuru,cezarsa/tsuru,gciotta/tsuru,cezarsa/tsuru,4eek/tsuru,cgvarela/tsuru,jblancett/tsuru,4eek/tsuru,gwmoura/tsuru,4eek/tsuru,RichardKnop/tsuru,jblancett/tsuru