Add another common issue to the README section

[luarx]

Is your feature request related to a problem? Please describe

I saw error message nginx: [emerg] mkdir() "/var/cache/nginx/client_temp" failed (30: Read-only file system) when trying to use the nginx unpriviledged image, and looking for the error message in this repo I could not find any information.

Describe the solution you'd like

I want to add this common issue to the README file (to this section)
As it is already explained in the nginx docker-library repo

Example:

• If you override the default nginx.conf file you may receive the message nginx: [emerg] mkdir() "/var/cache/nginx/client_temp" failed (30: Read-only file system), in this case you have to add these lines in the http context:

http {
    client_body_temp_path /tmp/client_temp;
    proxy_temp_path       /tmp/proxy_temp_path;
    fastcgi_temp_path     /tmp/fastcgi_temp;
    uwsgi_temp_path       /tmp/uwsgi_temp;
    scgi_temp_path        /tmp/scgi_temp;
...
}