rewrote GrAlphaThresholdFragmentProcessor in sksl

Bug: skia:
Change-Id: I641b206fc3bc19ac190ad94ee755ab9e1caab9b3
Reviewed-on: https://skia-review.googlesource.com/21341
Reviewed-by: Jim Van Verth <jvanverth@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
diff --git a/src/sksl/README b/src/sksl/README
index 0c1712d..c2718ad 100644
--- a/src/sksl/README
+++ b/src/sksl/README
@@ -75,8 +75,10 @@
 
   Supported section are:
     @header            (in the .h file, outside the class declaration)
+    @headerEnd         (at the end of the .h file)
     @class             (in the .h file, inside the class declaration)
     @cpp               (in the .cpp file)
+    @cppEnd            (at the end of the .cpp file)
     @constructorParams (extra parameters to the constructor, comma-separated)
     @constructor       (replaces the default constructor)
     @initializers      (constructor initializer list, comma-separated)