README.md: add compile notes
README.md: add compile notes

I came across this a second time, and it's still not quite obvious
how to compile libct right after doing git clone, so let's document it.

Signed-off-by: Kir Kolyshkin <c0159612b0ef32139322233232df1a0d189c4b9b@openvz.org>

hallyn/libct,xemul/libct,hallyn/libct,avagin/libct,avagin/libct,xemul/libct,xemul/libct,hallyn/libct,xemul/libct,avagin/libct