The easiest way is to output the variable delimiter ({{
) by using a variable expression:
{{ '{{' }}
Alternatives (used when you have to escape too much) are raw (verbatim since 1.12) blocks:
{% raw %}
<ul>
{% for item in seq %}
<li>{{ item }}</li>
{% endfor %}
</ul>
{% endraw %}
Actually, it’s quite well documented.