Update README with optional database dependencies
Update README with optional database dependencies

cinnamoncoin/eloipool-1,ahmedbodi/SHA-Eloipool-MMCFE-,uingei/eloi,thrasher-/eloipool,thrasher-/eloipool,pointgaming/point-gaming-btc,jameshilliard/4tar-eloipool,uingei/eloipool,wizkid057/eloipool,svasva/eloipool,thrasher-/eloipool,imsaguy/eloipool