Update CONTRIBUTING.md to reflect recent changes

[darkmyst]

The current CONTRIBUTING.md instructs one to execute the following:

npm install
npm run typings

but in a recent commit (ac5c7f9) the typings script was removed.

I'd like to experiment with this project and maybe eventually contribute, but I'm new to node library development and ng2 and I'd greatly appreciate it if these instructions could be updated to reflect current processes.

Great work on this library, I'm excited for the hopefully-forthcoming v3-beta!

[robwormald]

Please check this against NgRx v4, and if it’s still an issue, please reopen on https://github.com/ngrx/platform. Thanks!