[wiki/MUTN] Add shell completions troubleshooting guide

wiki/MUTN/completions.md

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+### ZSH completions not working?
+Make sure your ~/.zshrc contains the following:
+```shell
+autoload -Uz compinit && compinit
+```
+...and then restart your shell.
+***
+### Bash completions not working? 
+Install your distribution's 'bash-completion' package or source the completion script manually.
+
+For most environments, this would mean adding the following to your ~/.bashrc:
+```shell
+source /usr/share/bash-completion/completions/mutn
+```
+Note that this directory is different on FreeBSD.
+
+FreeBSD:
+```shell
+source /usr/local/share/bash-completion/completions/mutn
+```
+...and then restart your shell.
+
+*Please note that Bash completions are slightly more limited than ZSH completions, and as such, new entries will not be auto-completed until the completions script is re-sourced.*
+***
+### PowerShell completions not working?
+Ensure you are using modern PowerShell (7+) as opposed to the built-in Windows PowerShell.
+
+Currently, completions must be manually installed and sourced in your PowerShell profile as such:
+```shell
+Set-PSReadlineKeyHandler -Key Tab -Function Complete  # this line is optional; it makes tab completion function more similarly to Bash/ZSH
+. <drive letter>:\path\to\mutn.ps1
+```