Computes the normalized term frequency as explained in http://www.tfidf.com/

        :type sentences: [sumy.models.dom.Sentence]

def compute_tf(self, sentences):
        """
        Computes the normalized term frequency as explained in http://www.tfidf.com/

        :type sentences: [sumy.models.dom.Sentence]
        """
        content_words = self._get_all_content_words_in_doc(sentences)
        content_words_count = len(content_words)
        content_words_freq = self._compute_word_freq(content_words)
        content_word_tf = dict((w, f / content_words_count) for w, f in content_words_freq.items())
        return content_word_tf