Update tutorial

Add a section drawing attention to the risks of Markov text generators and some ways of mitigating them.

README.md

@@ -17,6 +17,13 @@ Your Markov chain grammar needs something to imitate.
 Paste some text in the input text box, or upload some plain text files.
 The output generated by the Tracery grammar will look like a mashed-up version of the input text.
 
+Be careful with texts containing words or phrases relating to sensitive topics.
+Your resulting text generator will be able to output every word in the input text,
+and almost every combination of concepts will be juxtaposed.
+To avoid producing any offensive or otherwise unwanted output,
+it's recommended to edit out sections of the source text as necessary,
+and check some of the generator's output before publishing it.
+
 You can use most characters in the input text,
 but `{` and `}` can cause Cheap Bots, Done Quick! to break,
 and unicode characters might not work when added with the file uploader.