Update battery threshold limit on Ideapad based on manufacturer settings (#838)

* idapad_fixed_threshold_manufacturer

* Update README.md

This is a specific config concerning model laptops with battery threshold limit value set by the manufacturer.

---------

Co-authored-by: autofreq <email@autofreq>

README.md

@@ -600,6 +600,31 @@ this works only with `lenovo_laptop` kernel module compatable laptops.
 
 add `ideapad_laptop_conservation_mode = true` to your `auto-cpufreq.conf` file
 
+### Special cases of Lenovo_ideapad (or some other models with fixed threshold)
+
+As you may know, for some laptop models you can only decide to limit battery charging but can not set the limit value. The limit value is set by the manufacturer in the system (generally 60% and sometimes 80%). Also, you can not set the value of start charging.
+
+This limit value is not always accessible for users to avoid changing it, but you can try looking in some of these paths : 
+
+```
+cat /sys/bus/platform/drivers/ideapad_acpi/VPC2004:00/charge_control_end_threshold
+cat /sys/class/power_supply/BAT0/charge_control_end_threshold
+cat /sys/class/power_supply/BAT0/charge_control_start_threshold
+```
+
+This is the config to apply at /etc/auto-cpufreq.conf in order to stop battery charging at 60% or 80% depending on the value set in the system by the manufacturer.
+
+```
+[battery]
+enable_thresholds = true
+start_threshold = 20
+stop_threshold = 1
+
+```
+start_threshold = 20 (should be present with a valid number but it's ignored)
+
+stop_threshold = 1 (to stop charging the battery at the limit value 60% or 80%)
+
 ### Ignoring power supplies
 
 you may have a controler or headphones and when ever they may be on battery they might cause auto-cpufreq