Add Caveats section to readme

Users that are unfamiliar with the project may find some aspects unintuitive.  We should try and warn them so that they can have a better experience 

These are all ways that users broke VVV while I was helping them at WordCamp Lancaster
Alter language a bit in common caveats section
Merge pull request #1 from jeremyfelt/patch-5

Alter language a bit in common caveats section
Merge pull request #291 from aaronjorbin/patch-4

Add Caveats section to readme