PERF: performance improvements in multi-key groupby
```
-------------------------------------------------------------------------------
Test name                                    | head[ms] | base[ms] |  ratio   |
-------------------------------------------------------------------------------
groupby_multi_index                          | 1008.6970 | 1861.1110 |   0.5420 |
-------------------------------------------------------------------------------
Test name                                    | head[ms] | base[ms] |  ratio   |
-------------------------------------------------------------------------------

Ratio < 1.0 means the target commit is faster then the baseline.
Seed used: 1234

Target [907e1b7] : performance improvements in multi-key groupby
Base   [5dc8009] : Merge pull request #9418 from sinhrks/rn_20150205

DOC: Fix release note format
```

pls add a release note in perf section, otherwise looks good.
 @jreback added the release note
 thanks!

we should prob rerun these benchmarks to see how much better things are now

https://github.com/Rdatatable/data.table/wiki/Benchmarks-%3A-Grouping
