Mention upload file size limit

Would be nice to mention the maxium upload file size on the page for user's convenience. It's already available as app.config['MAX_CONTENT_LENGTH'] in main.yml, so should be easy to implement. Simply pass this variable as max_content_length to render_template() and then the following should work:

Upload your file (max size: {{ (max_content_length | int) / 1024 / 1024 }}MB), get a <em>cleaned</em> one in response.