Every time I open a project that was already rendered I can not see the render layers in viewer node in the compositing. So I have to render them again and again every time I try to edit my compositing. Does Blender saves the rendering so I dont have to render layers all t he time to make changes in my compositing? I use cycles and blender 2.79
No, blender does not save the rendered layers as part of the project.
You need to render them again, or you can save them as an image first and import them back into blender for compositing.
To keep the best quality use a "output file node" and save as EXR multilayer. Read: Save all render passes to a single EXR image and Different render per layers? and How can I render all render passes to files in Cycles?