Add documentation about criteria page and adding to index

[clausmullie]

The readme could be updated with two steps for new incoming translations

1. Ensuring the criteria page has an index

{% assign sorted = site.pages | sort:"order" %}

{% for page in sorted %}{% if page.dir == "/iso/criteria/" %}{% if page.name != "index.md" %}{% if page.title %}
1. [{{page.title}}]({{site.baseurl}}{{page.url}}){% endif%}    {% endif%}  {% endif%}{% endfor %}

In which iso is updated for that particular translation

2. Listing the translation on the main index in alphabetical order