readme: linux kernel version group sections

Author: cirosantilli

README.adoc

@@ -2383,7 +2383,9 @@ On one hand, we would like to have our configs as a single git file tracked on t
 * reuse Buildroot's configs
 * split our configs into `min` and `default`
 
-=== Find the kernel version
+=== Kernel version
+
+==== Find the kernel version
 
 We try to use the latest possible kernel major release version.
 
@@ -2400,7 +2402,7 @@ cd linux
 git log | grep -E '    Linux [0-9]+\.' | head
 ....
 
-=== Update the Linux kernel
+==== Update the Linux kernel
 
 ....
 # Last point before out patches.
@@ -2440,7 +2442,7 @@ This backwards compatibility is just awesome, it makes getting and running the l
 
 This also makes this repo the perfect setup to develop the Linux kernel.
 
-=== Downgrade the Linux kernel
+==== Downgrade the Linux kernel
 
 The kernel is not forward compatible, however, so downgrading the Linux kernel requires downgrading the userland too to the latest Buildroot branch that supports it.