feat: Add support for fish shell in instructions

Add fish shell support in gitsign credential cache setup instructions.

Author: ananthb

Diff for: cmd/gitsign-credential-cache/README.md

@@ -192,6 +192,13 @@ if [ -f "${HOME}/.zshrc" ]; then
     shell_config_file="${HOME}/.zshrc"
 elif [ -f "${HOME}/.bashrc" ]; then
     shell_config_file="${HOME}/.bashrc"
+elif [ -f "${HOME}/.config/fish/config.fish" ]; then
+    if [ ! -f "${HOME}/.config/fish/conf.d/gitsign-credential-cache.fish" ]; then
+        echo "set -x GITSIGN_CREDENTIAL_CACHE \"${gitsign_cache_path}\"" > "${HOME}/.config/fish/conf.d/gitsign-credential-cache.fish"
+        echo "Added GITSIGN_CREDENTIAL_CACHE to ${HOME}/.config/fish/conf.d/gitsign-credential-cache.fish. Please restart your shell to apply the changes."
+    else
+        echo "GITSIGN_CREDENTIAL_CACHE already exists in ${HOME}/.config/fish/conf.d/gitsign-credential-cache.fish!"
+    fi
 else
     echo "No .bashrc or .zshrc found in your home directory."
     exit 1
@@ -221,4 +228,4 @@ and of course if you would like to tail the logs of your service you can do so b
 
 ```sh
 tail -f /opt/homebrew/var/log/gitsign-credential-cache.log
-```
+```