Add --help usage to install.sh and exit when invalid arg / Update contributing docs

.github/PULL_REQUEST_TEMPLATE.md

@@ -4,9 +4,6 @@ https://github.com/docker/cli/blob/master/CONTRIBUTING.md
 
 ** Make sure all your commits include a signature generated with `git commit -s` **
 
-For additional information on our contributing process, read our contributing
-guide https://docs.docker.com/opensource/code/
-
 If this is a bug fix, make sure your description includes "fixes #xxxx", or
 "closes #xxxx"
 

README.md

@@ -66,3 +66,11 @@ ruling it out in the future.
 
 docker/docker-install is licensed under the Apache License, Version 2.0.
 See [LICENSE](LICENSE) for the full license text.
+
+## Contributing
+
+Make sure you have read and understood our [contributing
+guidelines](https://github.com/docker/cli/blob/master/CONTRIBUTING.md).
+
+**Make sure all your commits are signed off and include a signature generated
+with `git commit -s`.**

install.sh

@@ -107,6 +107,17 @@ fi
 
 mirror=''
 DRY_RUN=${DRY_RUN:-}
+
+# Provide a helpful usage statement when --help or any invalid argument is passed
+# to the script. Exit code deliberately not included here as error depends on
+# argument provided.
+usage() {
+	echo
+	echo "USAGE: "
+	echo "    ${0} [--channel <stable|test>] [--mirror <Aliyun|AzureChinaCloud>] [--version <VERSION>] [--dry-run] [--help]"
+	echo
+}
+
 while [ $# -gt 0 ]; do
 	case "$1" in
 		--channel)
@@ -124,8 +135,14 @@ while [ $# -gt 0 ]; do
 			VERSION="${2#v}"
 			shift
 			;;
+		--help)
+			usage
+			exit 0
+			;;
 		--*)
 			echo "Illegal option $1"
+			usage
+			exit 1
 			;;
 	esac
 	shift $(( $# > 0 ? 1 : 0 ))