Update README.md

Hi,
fedora yum is obsolite log time ago...
https://www.2daygeek.com/comparison-difference-between-dnf-vs-yum/

Works for fedora 31:

[root@Galleon ~]# uname -a
Linux Galleon 5.6.13-200.fc31.x86_64 #1 SMP Thu May 14 23:26:14 UTC 2020 x86_64 x86_64 x86_64 GNU/Linux
[root@Galleon ~]#

Make load_env_levels inline function

tcp_client.h bugfix for macOS and add tcp example 

-> fix the issue that the head is not compilable bc. of missing declaration
-> optimize if/else block in connection function. Else not needed bc. of break in if

Fix for issue #1627: Global get_level() and should_log()

* Added: `spdlog::get_level()` API function - like `logger::level()`, except for the name change
* Added: `spdlog::should_log()` API function - like `logger: should_log()`

Bump CMake requirement to 3.10, don't enable C language

spdlog doesn't seem to actually require the C language.
Not enabling it results in a significant initial configure time speedup.