js: Point to new location of Interface conversion details. (#623)

When the "See GopherJS' README for details." phrase was added in 88f12aa,
the JavaScript <-> Go conversion table was in README.
It was moved in c6e849f to the package comment of js package.

Fixes #622.

js/js.go

@@ -73,7 +73,7 @@ func (o *Object) Uint64() uint64 { return o.object.Uint64() }
 // Float returns the object converted to float64 according to JavaScript type conversions (parseFloat).
 func (o *Object) Float() float64 { return o.object.Float() }
 
-// Interface returns the object converted to interface{}. See GopherJS' README for details.
+// Interface returns the object converted to interface{}. See table in package comment for details.
 func (o *Object) Interface() interface{} { return o.object.Interface() }
 
 // Unsafe returns the object as an uintptr, which can be converted via unsafe.Pointer. Not intended for public use.