Include *.md files inside mkdocs/tests/integration into the tarball

Without these files, the tests are incomplete.

MANIFEST.in

@@ -1,5 +1,5 @@
 include README.md
 include LICENSE
-recursive-include mkdocs *.ico *.js *.css *.png *.html *.eot *.svg *.ttf *.woff *.woff2 *.xml *.mustache *mkdocs_theme.yml
+recursive-include mkdocs *.ico *.js *.css *.png *.html *.eot *.svg *.ttf *.woff *.woff2 *.xml *.mustache *mkdocs_theme.yml *.md
 recursive-exclude * __pycache__
 recursive-exclude * *.py[co]