README.md: Include more details for modules with custom kernels

Added details about loadable modules being disabled when using a custom
kernel with WSL2. While WSL2 can boot and run with the v6.6 configs and
module support disabled, this may limit functionality for some users,
such as running Docker.

Advised users to set any needed modules to built-in (=y) before building
their custom kernel. Suggested using the last v5.15 config to build the
v6.6 kernel for those who prefer the previous configuration setup.

Signed-off-by: Kelsey Steele <kelseysteele@microsoft.com>

Author: kelsey-steele

README.md

@@ -33,7 +33,13 @@ as follows:
 2. Modify WSL2 kernel configs (optional):  
    `$ make menuconfig KCONFIG_CONFIG=Microsoft/config-wsl`
 
-   Loadable module support is disabled when using a custom built kernel. Set any modules you want to be built-in before building.
+When using a custom-built kernel, loadable module support is disabled.
+Configurations set as modules (=m) won’t be available. Set any needed modules
+to built-in (=y) before building your custom kernel.
+
+If you prefer the old configuration before WSL introduced loadable module
+support, you can use the old config file from the linux-msft-wsl-5.15.y branch
+to build your v6.6 WSL2 kernel.
 
 3. Build the kernel using the WSL2 kernel configuration:  
    `$ make KCONFIG_CONFIG=Microsoft/config-wsl`