C++: Put a warning on the PointsTo library.

Author: geoffw0

cpp/ql/lib/semmle/code/cpp/pointsto/PointsTo.qll

@@ -19,6 +19,9 @@
  * `pointstoinfo` predicate determines the transitively implied points-to
  * information by collapsing pointers into equivalence classes. These
  * equivalence classes are called "points-to sets".
+ *
+ * WARNING: This library may perform poorly on very large projects.
+ * Consider using another library such as ??? instead.
  */
 
 import semmle.code.cpp.commons.File