Updating README.md to reflect IPv6 configuration on Linux

[alexander-potemkin]

as mentioned at #2577

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[alexander-potemkin]

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[jyyi1]

Thanks a lot for the contribution!

I have a couple of suggestions for this change. The Linux commands you've provided, while helpful, might not work for every user since different Linux distributions can have different ways of handling these configurations. To avoid potential issues, I recommend we remove the specific commands (especially the one related to sysctl.d).

Additionally, our long-term goal is to fully support both IPv4 and IPv6. Since disabling IPv6 is a temporary workaround, I think it's better to add this as a TODO item in the documentation (see my suggestion). This will signal that we're aware of the issue and plan to address it in the future, once we have the bandwidth.

Finally, I'd suggest putting the text in the Outline Client desktop README. This keeps the main document clean and ensures that users can find information directly relevant to their platform.