State dependency on Linux explicitly in README file.

[mmueller41]

MxTasking heavily depends on Linux-specific functions, libraries and behaviour but Linux is not even mentioned in the README.md. This unnecessarily confuses users of other operating systems who want to use MxTasking and instead of being told that they need Linux, are bombarded with compiler errors. The only vague hint to the dependency on Linux is, the stated dependency on libnuma.

Noting that MxTasking is written for Linux would also better indicate, that this library cannot be easily ported to other platforms, not even to those who adhere to the POSIX-standard, such as the *BSDs and macOS.

For example, the per-processor heap implementation assumes behaviour that is specific to libnuma only, and is prohibited by gcc's libc implementation and considered undefined behaviour by the standard. In particular, numa_free() used in the destructor of class ProcessorHeap accepts various pointers, especially those who were not returned by libnuma's alloc functions, which crashes the application when free() is used instead. This specific behaviour is actively exploited in the implementation of ProcessorHeap and is not portable.

Another example is the use of sched_getcpu() in mx::system::topology which is a Linux-specific function and not part of the POSIX-API.

So, please consider updating the README.md to include Linux as mandatory dependency or make MxTasking POSIX-compliant, i.e. remove the Linux dependencies.