Add note about --strict and --strict-markers to references

[DahlitzFlorian]

As discussed in #7233, I added a note to references stating the preference for --strict-markers.

• Allow maintainers to push and squash when merging my commits. Please uncheck this if you prefer to squash the commits yourself.
• Create a new changelog file in the changelog folder, with a name like <ISSUE NUMBER>.<TYPE>.rst
• Add yourself to AUTHORS in alphabetical order.

@nicoddemus I hope I did not forget anything and the proposed note is sufficient. Let me know if I need to change something.

[nicoddemus]

Thanks @DahlitzFlorian!

[asottile]