Add remark for Windows Defender exclusion

[simonsan]

* Add Windows Defender exclusions for the destination and/or rustic.exe (which would also speed up all operations)

Is this something rustic could do? Or can you give an short How-To (which we can add to the docu once it is ready, see #302 )? Sorry, I can't judge if this is trival - I didn't use Windows for a long time...

Exceptions can be added via powershell or by writing to the registry, both can be done by rustic I imagine but you'd need to run it as administrator. Perhaps it's best to just document how to do it manually:

Open an elevated powershell and run Add-MpPreference -ExclusionProcess "rustic.exe".

There's a mention in the restic FAQ but I don't think the exclusion is documented anywhere (perhaps in the forum?).

Edit: There's a few mentions in the forum, most threads refer to that thread: https://forum.restic.net/t/windows-defender-causes-10x-slowdown/925.

Originally posted by @ducalex in rustic-rs/rustic#635 (comment)