Document the reason for targeting C++14

[jonesmz]

I think it would be an improvement to write a short paragraph explaining why mqtt_cpp is for C++14, instead of either C++11, or C++17 (or whatever other version someone may decide they want to support).

I'm using C++17 in all of my code, and think it has a lot of very very nice improvements to C++.

Mqtt_cpp does not need to change to C++17, but I am curious why you support C++14.

[redboltz]

mqtt_cpp is originally focused on C++11. C++14 includes some kind of bug fix of C++11. The position of C++14 is minor fixed version of C++11. I migrate to C++14.

C++17 includes big changes. I use C++17 my MQTT broker (proprietary project). But I use only C++14 for SDKs (proprietary project, kind of mqtt_cpp wrapped client library). SDK is integrated with user's code. Some of users (not so small amount) still uses C++14. They are not ready to use C++17.

I think that it is too early to enforce to use C++17 for users.

[jonesmz]

I think this should be part of readme.md