Add section about missing 'tools.jar' on Linux systems

I noticed that in section "Pull the libraries" there is described to build the IDE in the following way:

```
./build-all.sh -P scala-2.10.x clean install
```

Does that work for Kepler too? For me it didn't - I had to add a `-Peclipse-kepler`. Is this correct or went something else wrong without that option?