Improve the readme file

Hello @pierrrrrrre,

I made a few edits to the README file that I think makes it read better. 

For example, I changed the headings a bit. The title is H1 while each sub heading is H2 or H3, this is the convention used when handling headings. Also I converted the Title heading markup, `=====` to the markup `#` for uniformity. 

Another edit would be the removal of pronouns. "**You** do this by going to [...]", "Then **I** will get a file named [...]". Avoiding the use of pronouns is better for documentation. A side effect of removing pronouns is rewritten sentences and using the proper noun of the program when needed (using PyGeoIpMap instead of *it*).

There are a few other edits as well.  
This works but starting with Ubuntu 14.04 you don't need to specify `apt-get` anymore (`apt` is enough).