Document Requirement for using QUESO GetPot

[pbauman]

There was a behavior change after the 0.56.1 release. Looks like it happened in #471. In particular, we need to make that the user knows to #include <queso/Defines.h> and to include it before #include <queso/getpot.h>. The errors will be particularly confusing if the user already has #include <queso/getpot.h> because they'll get compiler errors that GetPot is undefined even though if you look at the preprocessed source (yes, I did), the header got parsed and included.

For sure will want this detail in the change log and the announcement for the next version.

[dmcdougall]

And I'll add an example too so I can point people to code.