doc/logitech: add HID++1.0 battery level info

This should help in https://github.com/pwr/Solaar/issues/49. This information
was shared by Nestor Lopez Casado from Logitech.

What remains unclear are the 0x25 and 0x26 values for r1 (Charging state). It is
also not clear whether r0 (Battery/Charging level) always reports 0x00 while
charging the Performance MX. (Perhaps this is only reported the first few
seconds?)

docs/logitech/battery-level.txt

@@ -0,0 +1,26 @@
+The battery/charging level and status is reported only if the related
+reporting flag in register 0x00 is enabled by the host. The
+"Battery/Charging Level" byte indicates the battery level if the
+"Charging State" indicates 0x00 ("Not Charging"). If "Charging State"
+indicates 0x21 to 0x23 ("Charging"), the "Battery/Charging Level" byte
+indicates the level of charging.
+
+10 ix 07 r0 r1 r2 00
+r0 -> Battery/Charging Level
+                  0x00 = Reserved/Unknown
+                  0x01 = Critical
+                  0x02 = Critical (legacy value, don't use)
+                  0x03 = Low
+                  0x04 = Low (legacy value, don't use)
+                  0x05 = Good
+                  0x06 = Good (legacy value, don't use)
+                  0x07 = Full
+                  0x08..0xFF = Reserved
+r1 ->  Charging state
+                  0x00 = Not charging
+                  0x01..0x1F = Reserved (not charging)
+                  0x20 = Unknown charging state
+                  0x21 = Charging
+                  0x22 = Charging complete
+                  0x23 = Charging error
+                  0x24..0xFF = Reserved