Add documentation for system profiles, snapshots & updating

doc/updating/Updatig.md

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+# Slic3r PE 1.40 configuration update
+
+Slic3r PE 1.40.0 comes with a major re-work of the way configuration presets work.
+There are three new features:
+
++ A two-tier system of presets being divided into _System_ and _User_ groups
++ Configuration snapshots
++ Configuration updating from the internet
+
+## System and User presets
+
+- _System preset_: These are the presets that come with Slic3r PE installation. They come from a vendor configuration bundle (not individual files like before). They are **read-only** – a user cannot modify them, but may instead create a derived User preset based on a System preset
+- _User preset_: These are regular presets stored in files just like before. Additionally, they may be derived (inherited) from one of the System presets
+
+A derived User preset keeps track of wich settings are inherited from the parent System preset and which are modified by the user. When a system preset is updated (either via installation of a new Slic3r or automatically from the internet), in a User preset the settings that are modified by the user will stay that way, while the ones that are inherited reflect the updated System preset.
+
+This system ensures that we don't overwrite user's settings when there is an update to the built in presets.
+
+Slic3r GUI now displays accurately which settings are inherited and which are modified.
+A setting derived from a System preset is represeted by green label and a locked lock icon:
+
+![a system setting](setting_sys.png)
+
+A settings modified in a User preset has an open lock icon:
+
+![a user setting](setting_user.png)
+
+Clickign the open lock icon restored the system setting.
+
+Additionaly, any setting that is modified but not yet saved onto disk is represented by orange label and a back-arrow:
+
+![a modified setting](setting_mod.png)
+
+Clicking the back-arrow restores the value that was previously saved in this Preset.
+
+## Configuration snapshots
+
+Configuration snapshots can now be taken via the _Configuration_ menu.
+A snapshot contains complete configuration from the point when the snapshot was taken.
+Users may move back and forth between snapshots at will using a dialog:
+
+![snapshots dialog](snapshots_dialog.png)
+
+
+# Updating from the internet
+
+Slic3r PE 1.40.0 checks for updates of the built-in System presets and downloads them.
+The first-time configuration assistant will ask you if you want to enable this feature - it is **not** mandatory.
+
+Updates are checked for and downloaded in the background. If there's is an update, Slic3r will prompt about it 
+next time it is launched, never during normal program operation. An update may be either accepted or refused.
+Before any update is applied a configuration snapshot (as described above) is taken.