[SPARK-1753] Warn about PySpark on YARN on Red Hat

In #30, both @tgraves and I ran into the issue of building the assembly jar on a Red Hat system: the resulting jar does not load the python files properly, which were needed for running PySpark on YARN.

In the medium term, we should figure out what the issue is, since Red Hat is used quite commonly. For now, we should at the very least document it so people don't run into the same headaches that plagued us for days.