Add fish shell completion hint to installer

See original request in
https://github.com/bazelbuild/bazel/pull/12249#discussion_r504056101

Closes #12267.

PiperOrigin-RevId: 337467923

scripts/packages/template_bin.sh

diff --git a/scripts/packages/template_bin.sh b/scripts/packages/template_bin.sh
index 420c1f4..7bcd518 100755
--- a/scripts/packages/template_bin.sh
+++ b/scripts/packages/template_bin.sh
@@ -178,10 +178,15 @@
 
 Bazel is now installed!
 
-Make sure you have "${bin}" in your path. You can also activate bash
-completion by adding the following line to your ${BASHRC}:
+Make sure you have "${bin}" in your path.
+
+For bash completion, add the following line to your ${BASHRC}:
   source ${base}/bin/bazel-complete.bash
 
+For fish shell completion, link this file into your
+${HOME}/.config/fish/completions/ directory:
+  ln -s ${base}/bin/bazel.fish ${HOME}/.config/fish/completions/bazel.fish
+
 See http://bazel.build/docs/getting-started.html to start a new project!
 EOF
 exit 0