Linux installation tutorial: make -j8

Added `-j8` flag to make in the tutorial.
I feel it's quite important for new users, who may otherwise spend a whole day just building the code!

Screenshot from built docs:
![doc_screenshot](https://f.cloud.github.com/assets/1893429/2157899/acb26e30-9488-11e3-98e2-502c51c941ac.png)