kernel: add a PATCHES text file describing the new patch categories

SVN-Revision: 26689

target/linux/generic/PATCHES

@@ -0,0 +1,15 @@
+The patches-* subdirectories contain the kernel patches applied for every
+OpenWrt target. All patches should be named 'NNN-lowercase_shortname.patch'
+and sorted into the following categories:
+
+0xx - upstream backports:
+1xx - code awaiting upstream merge
+2xx - kernel build / config / header patches
+3xx - architecture specific patches
+4xx - mtd related patches (subsystem and drivers)
+5xx - filesystem related patches
+6xx - generic network patches
+7xx - network / phy driver patches
+8xx - other drivers
+9xx - uncategorized other patches
+