Update install.md
Clarify that bash_completion script is already installed when installing bazel from the custom APT repository.
Closes
@@ -32,6 +32,9 @@ For more information on using Bazel, see
## <a name="bash"></a>Getting bash completion
+**Note:** If you installed Bazel using the custom APT repository, the bash completion script
+is already installed in `/etc/bash_completion.d`.
+
Bazel comes with a bash completion script, which the installer copies into the
`bin` directory. If you ran the installer with `--user`, this will be
`$HOME/.bazel/bin`. If you ran the installer as root, this will be
