Improve fragments documentation

See hakimel/reveal.js#2927 for a full example of custom effects.

I also added `current-visible`, a synonym to `fade-in-then-out`.

src/fragments.md

@@ -37,6 +37,7 @@ The default fragment style is to start out invisible and fade in. This style can
 | fade-left | Slide left while fading in |
 | fade-right | Slide right while fading in |
 | fade-in-then-out | Fades in, then out on the next step |
+| current-visible | Fades in, then out on the next step |
 | fade-in-then-semi-out | Fades in, then to 50% on the next step |
 | grow | Scale up |
 | semi-fade-out | Fade out to 50% |
@@ -50,6 +51,7 @@ The default fragment style is to start out invisible and fade in. This style can
 | highlight-current-blue | Turn text blue, then back to original on next step |
 {.key-value}
 
+Custom effects can be implemented by defining CSS styles for `.fragment.effectname` and `.fragment.effectname.visible` respectively, because the class `visible` is added to the fragment.
 
 ## Nested Fragments