README.md: Update reference to linux SubmittingPatches
README.md: Update reference to linux SubmittingPatches

The wise linux gods went an moved the content. May as well point to the
actual content.

Signed-off-by: Cyril Bur <6ce38dade8c4b52fd55c193fae70fd1d1275c3c3@au1.ibm.com>
Signed-off-by: Stewart Smith <ec31ab75ddf977353c8f660f92ea8b23f64aef25@linux.vnet.ibm.com>

shenki/skiboot,shenki/skiboot,shenki/skiboot,open-power/skiboot,legoater/skiboot,shenki/skiboot,mikey/skiboot,mikey/skiboot,shenki/skiboot,open-power/skiboot,qemu/skiboot,stewart-ibm/skiboot,qemu/skiboot,open-power/skiboot,qemu/skiboot,legoater/skiboot,stewart-ibm/skiboot,qemu/skiboot,mikey/skiboot,open-power/skiboot,legoater/skiboot,stewart-ibm/skiboot,qemu/skiboot,legoater/skiboot,legoater/skiboot,open-power/skiboot