code: public AVQuery<T> whereContains(String key, String substring) {
      conditions.whereContains(key, substring);
      return this;
    }
docs: Add a constraint for finding string values that contain a provided string. This will be slow
  for large datasets.
  
  @param key The key that the string to match is stored in.
  @param substring The substring that the value must contain.
  @return Returns the query, so you can chain this call.