Add justbuild to the list of clients and servers

[oreiche]

It officially supports the protocol in its client and server implementations.

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[mostynb]

I wonder if it's worth mentioning the need for the –compatible flag here? Just users might find this confusing otherwise.

[oreiche]

I wonder if it's worth mentioning the need for the –compatible flag here? Just users might find this confusing otherwise.

Agreed, I added a note in parentheses to both list entries.