README - add a flake8 project badge

Add a project badge image and display on the README.
This gives people a way to show they use Flake8
as the linter for their project,
and link to the Flake8 project page.

This may be a fun way for people to show their support,
and help drive new users to Flake8.

README.rst

@@ -2,6 +2,9 @@
  Flake8
 ========
 
+.. image:: docs/source/img/linter-flake8-green.svg
+    :target: https://gitlab.com/pycqa/flake8
+
 Flake8 is a wrapper around these tools:
 
 - PyFlakes

docs/source/img/linter-flake8-green.svg

@@ -0,0 +1 @@
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