Split documentation to multiple files in docs/ and use markdown

docs/internals/publishing.md

@@ -0,0 +1,7 @@
+# Publishing the package
+
+A small note on how the linter is usually published to PyPi:
+
+- `python3 setup.py sdist bdist_wheel --universal`
+- `twine upload dist/django_migration_linter-X.Y.Z-py2.py3-none-any.whl dist/django-migration-linter-X.Y.Z.tar.gz`
+