Cite shell parameter expansion
Cite shell parameter expansion

This is part of Issue #54.

In this commit we mention the shell parameter expansion to make
easy to students go deep if they need.

We also add a note about use ${some-var} instead of $some-var.

selimnairb/2014-02-25-swctest,selimnairb/2014-02-25-swctest,selimnairb/2014-02-25-swctest,selimnairb/2014-02-25-swctest,selimnairb/2014-02-25-swctest,selimnairb/2014-02-25-swctest