re-add opensearch.xml

Before:

![screen shot 2014-12-14 at 4 20 17 pm](https://cloud.githubusercontent.com/assets/87619/5534070/7a0a650e-8a0b-11e4-9963-9936cacc4afd.png)

After:

![screen shot 2014-12-22 at 6 49 21 pm](https://cloud.githubusercontent.com/assets/87619/5534071/891541c2-8a0b-11e4-82ae-42d64f644344.png)
![screen shot 2014-12-22 at 6 50 15 pm](https://cloud.githubusercontent.com/assets/87619/5534072/8df054a2-8a0b-11e4-82e2-cf919fdb55ff.png)


I stuck it in `assets/misc` since it seemed like the right place for it
npmjs.com is now the canonical domain.