opengl - Deferred shading - how to combine multiple lights?

I'm starting out with GLSL and I've implemented simple deferred shading that outputs G-buffer with positions, normals and albedo.

I've also written a simple point light shader.

Now I draw a sphere for the point light and output goes into a lighting buffer.

The problem is, how do I combine the results of lighting buffer when drawing multiple lights?

E.g. when I'm drawing the second light to the lightbuffer using the point light shader, how do I add first light to the second light in the lighting buffer. I mean, you can't read from and write to the same output buffer?

Answer

Additive blending, i.e. glBlendFunc(GL_ONE, GL_ONE) and glEnable(GL_BLEND).