gt510: add note about s2idle being fragile

systems/tablet_samsung_gt510/readme.md

@@ -37,4 +37,6 @@
 - see also https://wiki.postmarketos.org/wiki/Samsung_Galaxy_Tab_A_9.7_2015_(samsung-gt510)
 - for changing the kernel config via the pmbootstrap framework the following command should be used as the msm8916 kernel can be built for armv7l or aarch64: pmbootstrap kconfig edit --arch aarch64 postmarketos-qcom-msm8916
 - if the usb port in the kernel is switched from "peripheral" to "host" mode in the dtb (which is the case for latest kernel used here) it is possible to connect usb devices (keyboard, mouse, ethernet etc.) to it via a powered otg usb hub ... it has to be powered as the usb port itself will not provide power ... it looks like the tablet even gets powered well via the powered hub in this mode
-- suspend via s2idle mode seems to work quite well and the tablet should be able to survive more than a day in this mode - wakeup can be done via a short press on the power button or by conneting power to usb - sadly it seems to wake up on its own after about 30-60 minutes for yet unknown reasons
+- suspend via s2idle mode seems to work quite well and the tablet should be able to survive more than a day in this mode - wakeup can be done via a short press on the power button or by conneting power to usb
+  - sadly it seems to wake up on its own after about 30-60 minutes for yet unknown reasons
+  - the whole suspend/resume topic seems to be still very fragile here as just upgrading from v5.18.1 to v5.18.10 results in suspend working the first time, but waking up immediately again at the second suspend try