docs: add breaking change notice to postinstall

[mikehardy]

As discussed with you guys, and paired with the docs PR here invertase/react-native-firebase-docs#187 - I want to give the installed base a big heads-up that we may issue breaking releases in v5 minor releases during the transition to v6

I don't intend to propose anything crazy but sometimes maintenance requires a break when you are dependent on underlying libraries out of your control, and this gives us the space to break if we need to

This completes the effort to notify - we notify on package install, in the install docs, and in the release notes now, no one can say we didn't try, and the community should be able to self-resolve issues so it doesn't cause a support burden

Checklist

• Updated the documentation in the docs repo

Test Plan

I worked on it until it displayed correctly on install, it looks like this:

Release Plan

This can be merged any time I believe, assuming consensus on the plan to allow breaking change in the minor version semver space

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[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (v5.x.x@c81a539). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             v5.x.x    #2214   +/-   ##
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  Files             ?       64           
  Lines             ?     1589           
  Branches          ?        0           
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  Hits              ?     1156           
  Misses            ?      433           
  Partials          ?        0

[codecov]

Codecov Report

Merging #2214 into v5.x.x will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           v5.x.x    #2214   +/-   ##
=======================================
  Coverage   72.74%   72.74%           
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  Files          64       64           
  Lines        1599     1599           
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  Hits         1163     1163           
  Misses        436      436

[mikehardy]

The allow edits from maintainers thing is ticked, maybe something about the in-review flow doesn't allow commits? anyway I just accepted that wording change - looks good

[mikehardy]

I had to force-push to kick CI in the pants, but it's the same exact bits, and this is the last PR I wanted to push through right now, and I'll defer all the rest till after 5.4.3 so this will close the release. Can you approve me @Salakar :-) 🙏

[Salakar]

We've lost the comment tweaks I did above previously =D

[mikehardy]

We've lost the comment tweaks I did above previously =D

well that's not okay! sorry about that. I re-changed it to match your suggestion