docs: remove stray paragraph in notifications

[piksel]

the paragraph from the slack documentation were originally it's own section in the readme:

watchtower/README.md

Lines 406 to 409 in 985a922

	## Updating Watchtower
	If watchtower is monitoring the same Docker daemon under which the watchtower container itself is running (i.e. if you volume-mounted _/var/run/docker.sock_ into the watchtower container) then it has the ability to update itself. If a new version of the _containrrr/watchtower_ image is pushed to the Docker Hub, your watchtower will pull down the new image and restart itself automatically.

I added it back as well, since it could still be a useful section to have and it's definitely something that's missing from the docs right now. Could do with some more explaining and tips (pinning older version?)

fixes #946

[codecov]

Codecov Report

Merging #949 (cbc7f96) into main (6b155a1) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #949   +/-   ##
=======================================
  Coverage   54.47%   54.47%           
=======================================
  Files          25       25           
  Lines        1452     1452           
=======================================
  Hits          791      791           
  Misses        594      594           
  Partials       67       67           

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 91bbe74...cbc7f96. Read the comment docs.