[5.4] Docs: Add a PR targeting section to the README

[LadySolveig]

Issue # .

Summary of Changes

Adding a clear guidance to the README.md for contributors on which branch to target when submitting pull requests.
The new section uses a simple classification table to help contributors choose the correct branch based on the type of change.

Testing Instructions

Actual result BEFORE applying this Pull Request

Expected result AFTER applying this Pull Request

Link to documentations

Please select:

• Documentation link for docs.joomla.org:

• No documentation changes for docs.joomla.org needed

• Pull Request link for manual.joomla.org:

• No documentation changes for manual.joomla.org needed

[brianteeman]

Shouldn't the same change also be made in readme.txt?

[LadySolveig]

Good question, they are definitely not the same texts, the txt seems to have a few more passages and probably hasn't been updated for a while.

No, sorry, I was wrong, in the 6 branch it is also written on 6 so it is current.

[LadySolveig]

The How to get a working installation from the source section is also not in the txt-file.
I don't think it's the idea that they are identical, but I can't say for sure. I think that since it is primarily intended to make collaboration here on Github easier and transparent, the .md file is definitely not wrong.

[chmst]

I have tested this item ✅ successfully on 863fcda

Good Idea.

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