Added one more question to the FAQ.

That would be, in README.md.
Moved jsoncpp out of the src folder.

- The jsoncpp folder is now in the top level directory. It contains a
CMakeLists.txt file which compiles it.
- Also, the CMakeLists.txt in src/common and the root directory have both
been updated to reflect the change.
- Bonus: Extra comments added to /CMakeLists.txt.