Update README.md: Getting started

[RobbertH]

Moving the gcc linker option -ldl later.
With gcc -ldl demo.c -o demo, I got:

/usr/bin/ld: /tmp/ccg6cKgo.o: in function `main':
demo.c:(.text+0x30): undefined reference to `dlopen'
/usr/bin/ld: demo.c:(.text+0x40): undefined reference to `dlerror'
/usr/bin/ld: demo.c:(.text+0x6d): undefined reference to `dlerror'
/usr/bin/ld: demo.c:(.text+0x80): undefined reference to `dlsym'
/usr/bin/ld: demo.c:(.text+0x89): undefined reference to `dlerror'
/usr/bin/ld: demo.c:(.text+0x11d): undefined reference to `dlclose'
collect2: error: ld returned 1 exit status

While gcc demo.c -ldl -o demo compiles.
Here's a stackoverflow answer that explains why this is happening:
https://stackoverflow.com/questions/11893996/why-does-the-order-of-l-option-in-gcc-matter/11894098#11894098

[cgaebel]

Thank you!