README: explicitly document compiler requirements

README: explicitly document compiler requirements

Document C11 + GNU extensions as the requirement, omitting the exact
list of extensions since it's quite a bit.

The minimum compiler versions are somewhat arbitrary - we don't check
for them, so your experience may vary. Patches to update the README, or
to make kmod more widely buildable are welcome.

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Link: https://github.com/kmod-project/kmod/pull/359
Signed-off-by: Lucas De Marchi <lucas.de.marchi@gmail.com>