docs/installation: typo fix, clarification

 - Fix typo in rules path
 - Fix capitalisation.
 - `udevadm control --reload-rules` is unnecessary[1].
 - `adduser` is Debian-specific, use the more portable `gpasswd`.
 - No need to wait 10 seconds before re-inserting.
 - After adding to group, user must re-login.

 [1]: http://unix.stackexchange.com/a/39485/8250
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?)
hid10: update 07 battery parsing based on spec

This fixes pwr/Solaar#49 partially, at least the charging state will be reported
correctly hereafter. The charge level may still be incorrect though.
Merge pull request #54 from Lekensteyn/doc-updates

Doc updates