Add descriptions of native execution

Since a previous commit modified the rules for determining when QEMU is
used, this commit adds related descriptions to the README to explain how
developers can make the build system verify whether native execution can
be performed.

Author: DrXiao

README.md

@@ -66,6 +66,11 @@ emulation on GNU/Linux:
 $ sudo apt-get install qemu-user
 ```
 
+The build system is able to verify whether the running machine can perform native
+execution without QEMU. The host machine may install the prebuilt
+[fastfetch](https://github.com/fastfetch-cli/fastfetch/), which allows the build
+system to determine whether native execution can be enabled.
+
 It is still possible to build `shecc` on macOS or Microsoft Windows. However,
 the second stage bootstrapping would fail due to `qemu-arm` absence.