Update guides/CHANGELOG.md

[bogdanvlviv]

To Rails 5.2.0.beta1 was added "Threading and Code Execution in Rails" Guide
by 8bfa617.

To Rails 5.2.0.rc1 was added "Active Storage Overview" Guide
by 48fbc4a.

This commit adds changelogs about adding these guides.
I think it deserves to be mentioned inguides/CHANGELOG.md

[rails-bot]

r? @eileencodes

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[rails-bot]

Warning

• Pull requests are usually filed against the master branch for this repo, but this one is against 5-2-stable. Please double check that you specified the right target!

[kaspth]

Thanks, don't forget [ci skip] too :)