Add a note regarding using older kernels on Archlinux

README.md

@@ -26,6 +26,9 @@ Comes with a slightly modified Arch config asking for a few core personalization
 If you want to streamline your kernel config for lower footprint and faster compilations : https://wiki.archlinux.org/index.php/Modprobed-db
 You can optionally enable support for it at the beginning of the PKGBUILD file. **Make sure to read everything you need to know about it as there are big caveats making it NOT recommended for most users**.
 
+**Note regarding kernels older than 5.9 on Archlinux:**
+**Since the switch to zstd compressed initramfs by default, you will face an "invalid magic at start of compress" error by default. You can workaround the issue by editing `/etc/mkinitcpio.conf` to uncomment the `COMPRESSION="lz4"` (for example, since that's the best option after zstd) line and regenerating for all kernels with `sudo mkinitpcio -P`.**
+
 ## Other stuff included:
 - Graysky's per-CPU-arch native optimizations - https://github.com/graysky2/kernel_gcc_patch
 - memory management and swapping tweaks