Add documentation for --prepend-config option

docs/source/manpage.rst

@@ -80,6 +80,11 @@ All options available as of Flake8 3.1.0::
     --output-file=OUTPUT_FILE
                           Redirect report to a file.
     --tee                 Write to stdout and output-file.
+    --prepend-config=PREPEND_CONFIG
+                          Provide extra config files to parse in addition to the
+                          files found by Flake8 by default. These files are the
+                          first ones read and so they take the lowest precedence
+                          when multiple files provide the same option.
     --append-config=APPEND_CONFIG
                           Provide extra config files to parse in addition to the
                           files found by Flake8 by default. These files are the

docs/source/user/invocation.rst

@@ -130,6 +130,11 @@ And you should see something like:
       --output-file=OUTPUT_FILE
                             Redirect report to a file.
       --tee                 Write to stdout and output-file.
+      --prepend-config=PREPEND_CONFIG
+                            Provide extra config files to parse in addition to the
+                            files found by Flake8 by default. These files are the
+                            first ones read and so they take the lowest precedence
+                            when multiple files provide the same option.
       --append-config=APPEND_CONFIG
                             Provide extra config files to parse in addition to the
                             files found by Flake8 by default. These files are the

docs/source/user/options.rst

@@ -80,6 +80,8 @@ Index of Options
 
 - :option:`flake8 --tee`
 
+- :option:`flake8 --prepend-config`
+
 - :option:`flake8 --append-config`
 
 - :option:`flake8 --config`
@@ -739,6 +741,27 @@ Options and their Descriptions
         tee = True
 
 
+.. option:: --prepend-config=<config>
+
+    :ref:`Go back to index <top>`
+
+    Provide extra config files to parse in before and in addition to the files
+    that |Flake8| found on its own. Since these files are the first ones read
+    into the Configuration Parser, they have the lowest precedence if it
+    provides an option specified in another config file.
+
+    Typical use is to specify cross-project options that can still be
+    overridden by user or project config files.
+
+    Command-line example:
+
+    .. prompt:: bash
+
+        flake8 --prepend-config=my-default-config.ini dir/
+
+    This **can not** be specified in config files.
+
+
 .. option:: --append-config=<config>
 
     :ref:`Go back to index <top>`