add placeholder with notes on todo, workaround for #120

Tackling this properly requires proper handling of currying, see #122.
This is not a proper fix, since it doesn't yet understand the
placeholder. But right now the best we could do here is not error right
away.

index.d.ts

@@ -232,6 +232,19 @@ declare namespace R {
 
     interface Static {
 
+       /**
+        * A special placeholder value used to specify "gaps" within curried
+        * functions, allowing partial application of any combination of
+        * arguments, regardless of their positions.
+        * NOTE: can't type this yet, for binary functions consider R.flip!
+        */
+       __: any; // { "@@functional/placeholder": boolean };
+       // ^ don't type by value, since it can be inserted anywhere...
+       // Note this is still not useful, as it doesn't take into account how it changes formulas (leaving holes!).
+       // This remains TODO, and should be done on the level of curried functions, but that
+       // pretty much requires being able to express the separate functions as a single curried function...
+       // until that moment handling this would mean having to handle each possible combination for each function. :(
+
        /**
         * Adds two numbers. Equivalent to a + b but curried.
         */