Using React's Perf tools to profile rum apps

[lynaghk]

It took me a while to figure out how to get React's Perf tools working within ClojureScript, and I think it'd be helpful for others to have a section in the Rum README.
I've enclosed a draft below.

I'm not sure the best way to configure Leiningen / cljsbuild / figwheel to keep the Perf tools only in development builds (or if they'll get advanced-optimizationed away) --- I bet it could be done with Leiningen's project.clj :profiles key, but there may be a better way.
I'd love if someone could chime in on that.

## Profiling

To use [React.js Perf tools](https://facebook.github.io/react/docs/perf.html) from Rum, specify the `react-with-addons` dependency in your `project.clj`, making sure to exclude any transitive react dependencies:

```clj
[rum "0.9.0" :exclusions [cljsjs/react cljsjs/react-dom]]
[cljsjs/react-dom "15.1.0-0" :exclusions [cljsjs/react]]
[cljsjs/react-dom-server "15.1.0-0" :exclusions [cljsjs/react]]
[cljsjs/react-with-addons "15.1.0-0"]
```

Then from within your program run:

```clj
(js/React.addons.Perf.start)
;;run your app
(js/React.addons.Perf.stop)
(js/React.addons.Perf.printWasted)
```

and results will be printed to the developer console.

[piranha]

or if they'll get advanced-optimizationed away

I bet they won't be - Closure Compiler does not optimize :foreign-libs, which is how everybody uses React. It seems to me profiles are the best way to handle that.

[rauhs]

I think this would be great for the wiki. I just created a wiki for rum and added some useful links: https://github.com/tonsky/rum/wiki

[tonsky]

Moved to wiki