Serve locally instructions incomplete

[sverhoeven]

As a contributor to the tutorial I want to serve it locally. Several options are described

• In CONTRIBUTING.md it says to install Ruby with either OS package manager or conda and run make serve
• In .github/PULL_REQUEST_TEMPLATE.md it says you can also use make docker-serve.

Can the docker-serve option be added to CONTRIBUTING.md so both document give same options.

[sverhoeven]

Also using conda env create -f environment.yml -n esmvaltool_tutorial and make serve I got an error building native extension for eventmachine gem.

[SarahAlidoost]

There is another issue #44 and one PR #35 related to this one.