Added React London conference, March 28th

[StuartHarris]

Thanks :-)

Before submitting a pull request, please make sure the following is done...

1. Fork the repo and create your branch from master.
2. If you've added code that should be tested, add tests!
3. If you've changed APIs, update the documentation.
4. Ensure the test suite passes (npm test).
5. Make sure your code lints (npm run lint) - we've done our best to make sure these rules match our internal linting guidelines.
6. If you haven't already, complete the CLA.

[gaearon]

Thanks