docs(readme): improve installation instructions

README.md

@@ -64,15 +64,17 @@ It has been ported from POSIX shell to Tcl to Go and from [GPG](https://gnupg.or
 ## Installation
 
 You will need Go 1.22 or later to install pago.
-Once Go is installed on your system, run the following command:
+Once Go is installed on your system, run the following commands:
 
-```
+```shell
+# pago and pago-agent are separate binaries.
+# You should install both unless you have specific reasons not to.
 go install dbohdan.com/pago/cmd/pago@latest
 go install dbohdan.com/pago/cmd/pago-agent@latest
 ```
 
 Shell completion files for Bash and fish are available in [`completions/`](completions/).
-To install completions for fish, run `install.fish`.
+To install completions for fish, clone the repository and run `install.fish`.
 
 ## Supported platforms