Build requirements need to be better documented

Currently our documentation states:

Supported Platforms
===================

Windows XP SP2 or later
macOS 10.7 or later
Linux (tested on Debian 6/7 and Ubuntu 13)

Building the sources requires Qt 5.4.0 or later.

Build Instructions
==================
Prerequisites:
   * Qt 5.4.0 or later
   * On Windows:
     -  MinGW or Visual Studio
   * On macOS: Xcode

The installed toolchains have to match the one Qt was compiled with.

There is no information regarding what minimum compiler versions are required to build Qbs. We should document the minimum Visual Studio, MinGW, Xcode, clang, and gcc versions we have tested and know to be functional (these can probably be quite high, and do not necessarily have to be <= Qt's as Qbs will never be part of the Qt bootstrap process).

The "supported platforms" is probably fine as that mostly depends on the minimum Qt version, though the "tested on Debian 6/7 and Ubuntu 13" may need to be updated as Ubuntu 13 is several years old and this part of the README has not been updated since... 2013.

I believe the current minimum Visual Studio is 2013. My minimum Xcode recommendation would simply be "the latest version" and there's very little reliance on native SDK functionality so the build should work on versions far prior to anything anyone is likely to end up using.

Many Linux systems are horribly slow at updating, so documenting the minimum gcc/clang versions is especially useful there.