Add guidance about existing Vagrantfile.

See issue #37
Add hint about checking firewall settings.

This hit me and costed me approx. one hour :-(
Add link to example server.

This eases following the tutorial.
Fix typo.
Fix typo.
Solve a todo by pointing to next step (where it is solved).

See issue #41.
Cleanup useless markup.
Merge pull request #40 from htgoebel/master

Add guidance about existing Vagrantfile.