docs: Add documentation on the new -f colored option

docs/configuration.rst

@@ -102,8 +102,8 @@ Errors and warnings
 -------------------
 
 Problems detected by yamllint can be raised either as errors or as warnings.
-The CLI will output them (with different colors when using the ``standard``
-output format).
+The CLI will output them (with different colors when using the ``colored``
+output format, or ``auto`` when run from a terminal).
 
 By default the script will exit with a return code ``1`` *only when* there is one or
 more error(s).

docs/quickstart.rst

@@ -68,6 +68,10 @@ The output will look like (colors are not displayed here):
    10:1      error    too many blank lines (4 > 2)  (empty-lines)
    11:4      error    too many spaces inside braces  (braces)
 
+By default, the output of yamllint is colored when run from a terminal, and pure
+text in other cases. Add the ``-f standard`` arguments to force non-colored output.
+Use the ``-f colored`` arguments to force colored output.
+
 Add the ``-f parsable`` arguments if you need an output format parsable by a
 machine (for instance for :doc:`syntax highlighting in text editors
 <text_editors>`). The output will then look like: