Update Readme - socket.io 3.x support notice

[twofingerrightclick]

Inform users that socket.io 3.x is not supported yet.

[robbiedood]

Yes, put this notice in readme is really helpful. Otherwise, people would spent hours debugging and be misled.

((Hope to know if there is a short-term plan to support 3.x. ))

[IlyaDiallo]

Even if the client lib is updated in the future, upgrading Android clients in the wild will be a problem. The decision of breaking compatibility for 3.x was a sh*t move IMO.

[darrachequesne]

@twofingerrightclick thanks a lot for this, and sorry for the confusion.

@lukelu0520 yes indeed, that's next on my to-do list.

The decision of breaking compatibility for 3.x was a sh*t move IMO.

@IlyaDiallo those changes were necessary to address several shortcomings (security, performance and stability wise) that were identified over the last few years. That being said, I'm open to suggestions on how we could ease the migration.