I actually found it hard to come by good info when I was developing the feature too, especially since WebGL shaders, OpenGL ES shaders, and desktop OpenGL shaders are all different in subtle ways and often documentation is for one specific variant without saying so.
Also, Construct 2 only uses a small subset of shaders. While you can do a lot in general, C2 shaders are only a fragment shader with some built-in and custom parameters.
Learning by example is probably your best bet. Lots of pretty examples here: http://glsl.heroku.com/ (but some are pretty heavy on math!)