Clarify shell completions require manual setup

[udryan10]

Summary

• I found the shell completions docs confusing — I assumed running mux completions zsh would automatically add the completion source to my rc file. The previous wording ("add to ~/.zshrc") read more like a description of what the command does than an instruction to the user.
• Updated the README to make it explicit that you need to manually add the line to your shell config file, and added a concrete example of how to reload it.

Test plan

• Verify README renders correctly on GitHub

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