Add Benchmarks to Docs

[amilajack]

Would be awesome if some benchmarks could be added to show the difference between the webpack perf with and without hard source. Happypack has a good example

[mzgoddard]

Here is an old benchmark from Happypack's developer. Leaving it here mostly as a note for myself.

https://github.com/webpack/webpack/issues/250#issuecomment-241195440

Elapsed (ms)	Happy?	Hard-Sourced?	DLLs?
95451*	No	No	No
4155		Yes
32578	Yes	No
4354		Yes
90216	No	No	Yes
29077	Yes
4511		Yes

• I find this figure to be strange because normally I get much lower times (~60s without happypack and 10-11s with happypack), there may be something up with my box at the moment.