Provide a FAQ

[nlohmann]

As proposed on Reddit, it would be a good idea to have a FAQ section.

Possible questions:

• Why does the code not work with GCC 4.8? (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=55817, https://gcc.gnu.org/bugzilla/show_bug.cgi?id=57824)
• Is putting all code in a single header a good idea? (http://stackoverflow.com/questions/583255/c-code-in-header-files)
• Why are elements of objects ordered when I iterate/dump them?

[nlohmann]

This issue can be closed:

• The first element of the list is now part of the README file.
• The second element ("Is putting all code in a single header a good idea?") was actually never asked so far.
• The third element of the list is mentioned in the documentation of type object_t.

So far I do not think that a separate FAQ would make sense. Until it does, I shall try to move information to README.md, CONTRIBUTING.md, or the code documentation.