Change example from SiIV@4654 to SiIV@4116.
Add how to use NIST
Move Methodology to a file.
Describe better the docstring.

Gave a brief introduction and explained how to use the script.
Change mention of SiIV@4654 to SiIV@4116
Merge pull request #2 from gabraganca/example

Change spectral line from example.