docs: update README (#57)

* DOCS: update README

Add a "migrate from cmsplugin" management command mention in README

* docs: update README

Improve the "migrate from cmsplugin" mention, add a link to the
deprecate_cmsplugin_filer app.

Author: corentinbettiol

README.rst

@@ -98,3 +98,19 @@ You can run tests by executing::
     :target: https://www.djangoproject.com/
 .. |djangocms| image:: https://img.shields.io/badge/django%20CMS-3.7%2B-blue.svg
     :target: https://www.django-cms.org/
+
+
+Updating from `cmsplugin-filer <https://github.com/django-cms/cmsplugin-filer>`_
+--------------------------------------------------------------------------------
+
+Historically, `cmsplugin-filer` was used to create file, folder, image, link, teaser & video plugins on your django CMS projects. Now `cmsplugin-filer` has been archived, you can still migrate your old instances without having to copy them manually to the new `djangocms-<file|picture|link|...>` plugins.
+
+There's a third-party management command that supports your migration:
+
+`migrate_cmsplugin_filer.py <https://gist.github.com/corentinbettiol/84a6ea7e4d047fc01861b0af15fd60f0>`_
+
+This management command is only a starting point. It *has* worked out of the box for some people, but we encourage you to read the code, understand what it does, and test it on a development environment before running it on your production server.
+
+The management command is only configured to transfer your `cmsplugin_link`, `cmsplugin_file`, `cmsplugin_folder` and `cmsplugin_image` plugins to modern `djangocms_*` plugins. If you need to transfer other `cmsplugin_*` plugins, you'll have to write your own code.
+
+Alternatively you can use the `deprecate_cmsplugin_filer <https://github.com/ImaginaryLandscape/deprecate_cmsplugin_filer>`_ app, which only adds a small migration that transfer the old `cmsplugin-filer` plugins instances to the new `djangocms-<file|picture|link|...>` plugins.