[LTC] Update quickstart documentation
Suppose the following happens:
1. Do some development on master
2. Switch to lazy_tensor_staging and `python3 setup.py develop`
3. `cd lazy_tensor_core` and `python3 setup.py develop`
On step 3, it seems like the build will try to build/include/link with
the local installation of pytorch, which is still the version from
master.
Instead, in step 2, `python3 setup.py install` should be used instead of
`python3 setup.py develop`
