Readme improvement propositions

[cyrilhamidechi]

Hello,

First of all thanks for this very useful tool.

As user of this tool since few weeks now I'd like to submit few improvements about the readme:

• -c option is required for the -p option to work
• env > secrets-manager > key requires ARN instead of also supporting name (as in-cloud CodeBuild)
• Maybe a scripted quick install?

Quick install proposal

	git clone https://github.com/aws/aws-codebuild-docker-images.git
	cd aws-codebuild-docker-images/ubuntu/standard/3.0/
	docker build ./ -t aws/codebuild/standard:3.0
	docker pull amazon/aws-codebuild-local:latest --disable-content-trust=false
	cp codebuild_build.sh /usr/local/bin/codebuild_build

[awszhen]

Hi @cyrilhamidechi , these suggestions look good to me. Please submit pull request. Thanks!

[subinataws]

Closing this in favor of #252, which lists broader set of improvements. Feel free to submit a PR for this specific call out.