Update flatpak-warning.txt

[B-Teague]

Added a comment regarding branch name needing to match org.freedesktop.Sdk branch name.

I kept getting "extension not found" when trying to enabled the SDK and starting Code until I figured this out.

[flathubbot]

Started test build 113535

[flathubbot]

Build 113535 successful
To test this build, install it from the testing repository:

flatpak install --user https://dl.flathub.org/build-repo/111136/com.visualstudio.code.flatpakref

[nikodunk]

Do you want to add this to the project readme too @B-Teague , to which we directly link from various places like flathub/gnome software/ kde discover, etc?

[flathubbot]

Started test build 20966

[B-Teague]

Sorry for the long delay. I have also updated the readme

[flathubbot]

Build 20966 successful
To test this build, install it from the testing repository:

flatpak install --user https://dl.flathub.org/build-repo/3606/com.visualstudio.code.flatpakref

[flathubbot]

Started test build 20967

[flathubbot]

Build 20967 successful
To test this build, install it from the testing repository:

flatpak install --user https://dl.flathub.org/build-repo/3607/com.visualstudio.code.flatpakref