few typos and edit package section to adapt to next Rcpp

I assign the copyright of this contribution to Hadley Wickham. 

About the XXX note, I've tried to implement match like this: 

```cpp
// [[Rcpp::export]]
IntegerVector matchCpp( CharacterVector x, CharacterVector table){
  // first train an unordered_map
  std::unordered_map<String,int> map ;
  for( int i=0; i<table.size(); i++){
    String s = table[i] ;
    map[s] = i +1 ;
  }
  
  // now lookup
  int n = x.size() ;
  IntegerVector out(n) ;
  for( int i=0; i<n; i++){
    String s = x[i] ;
    out[i] = map[s]->second ;
  }
  return out ;
}
```

But the compiler complains about missing implementation of `std::hash<String>`. I've logged it as https://github.com/RcppCore/Rcpp/issues/84.