Make shell completion instructions more generic for other shells

For a while now we have supported the generation of argument completion
scripts for various shells. Yet, the README was only talking about the
usage of bash.
This change updates the document to at least mention the possibility of
creation of such completion scripts for other shells.

README.md

@@ -113,18 +113,23 @@ It is recommended that the resulting executable be installed in a
 directory accessible via the `PATH` environment variable.
 
 
-#### Bash Completion
-**nitrocli** comes with Bash completion support for options and
-arguments to them. A completion script can be generated via the
+#### Shell Completion
+**nitrocli** comes with completion support for options and arguments to
+them (for various shells). A completion script can be generated via the
 `shell-complete` utility program and then only needs to be sourced to
 make the current shell provide context-sensitive tab completion support.
 ```bash
 $ cargo run --bin=shell-complete bash > nitrocli.bash
 $ source nitrocli.bash
 ```
 
-The generated completion script can be installed system-wide as usual
-and sourced through Bash initialization files, such as `~/.bashrc`.
+The generated completion script (`bash` specific, in this case) can be
+installed system-wide as usual and sourced through Bash initialization
+files, such as `~/.bashrc`.
+
+Completion scripts for other shells work in a similar manner. Please
+refer to the help text (`--help`) of the `shell-complete` program for
+the list of supported shells.
 
 
 Known Problems