Allow disabling all checks for a file

Allow disabling of a file, even if it is invalid YAML (syntax error) by
including `# yamllint disable-file` in the first line.

docs/disable_with_comments.rst

@@ -73,3 +73,31 @@ It is possible, although not recommend, to disabled **all** rules:
 
 If you need to disable multiple rules, it is allowed to chain rules like this:
 ``# yamllint disable rule:hyphens rule:commas rule:indentation``.
+
+Disabling all checks for a file
+-------------------------------
+
+To prevent yamllint from reporting problems for a specific file, add the
+directive comment ``# yamllint disable-file`` as the first line of the file.
+For instance:
+
+.. code-block:: yaml
+
+ # yamllint disable-file
+ # The following mapping contains the same key twice, but I know what I'm doing:
+ key: value 1
+ key: value 2
+
+ - This line is waaaaaaaaaay too long but yamllint will not report anything about it.
+   This line will be checked by yamllint.
+
+or:
+
+.. code-block:: jinja
+
+ # yamllint disable-file
+ # This file is not valid YAML because it is a Jinja template
+ {% if extra_info %}
+ key1: value1
+ {% endif %}
+ key2: value2