I've an external monitor connected through HDMI and thinking about getting another external monitor for my MacBook Pro 2015.
How can I connect a second external display to my MacBook Pro?
You can use a MiniDisplay Port Cable for the second Monitor.
With an MacBook Pro Early 2015 you can connect two display using Thunderbolt 2
2x up to 3840x2160 on external displays via Thunderbolt 2
or one display using Thunderbolt 2 and a second one using HDMI
1x single display up to 3840x2160 via Thunderbolt 2
1x single 1080p display at up to 60 Hz, 3840x2160 at 30 Hz, or 4096x2160 at 24 Hz via HDMI
So in your case you need a Mini DisplayPort to HDMI adapter for a second HDMI display. If you want to use display with a higher resolution you need to get another adapter (DVI, DisplayPort, etc.).