title: I need to add specific rows in pandas DataFrame, at specific position
body: <p>I'm currently working on a project and I need to add specific rows whenever the tagged sentence ends. Whenever the 'N' column equals 1 it means that a new sentence started. I want to add two rows for each sentence: a row with 'Pos'= START at the beginning of the sentence, and a row with 'Pos'=End at the end of each row.
  This is what the DataFrame look like:</p>
  <pre><code>POSTAG = {
          'N': [1,2,3,4,5,6,7,8,9,10,11,12,13,14,1,2,3,4,5,6,7,8,9,1,2,3,4,5,6,7,8,9,10,11,1,2,3,4,5,6,7,8,9],
          'Name': ['ἐρᾷ','μὲν','ἁγνὸς','οὐρανὸς','τρῶσαι','χθόνα',',','ἔρως','δὲ','γαῖαν','λαμβάνει','γάμου','τυχεῖν','.','ὄμβρος','δ̓','ἀπ̓','εὐνάοντος','οὐρανοῦ','πεσὼν','ἔκυσε','γαῖαν','.','ἡ','δὲ','τίκτεται','βροτοῖς','μήλων','τε','βοσκὰς','καὶ','βίον','Δημήτριον','.','δενδρῶτις','ὥρα','δ̓','ἐκ','νοτίζοντος','γάμου','τέλειος','ἐστί','.'],
          'Pos': ['VERB','ADV','ADJ','NOUN','VERB','NOUN','PUNCT','NOUN','CCONJ','NOUN','VERB','NOUN','VERB','PUNCT','NOUN','ADV','ADP','ADJ','NOUN','VERB','VERB','NOUN','PUNCT','DET','ADV','VERB','NOUN','NOUN','ADV','NOUN','CCONJ','NOUN','ADJ','PUNCT','NOUN','NOUN','ADV','ADP','VERB','NOUN','ADJ','VERB','PUNCT']
          }
  
  df = pd.DataFrame(POSTAG, columns = ['N', 'Name','Pos'])
  print (df)
  </code></pre>
  <p>In this case I need a [Nan, Nan, START] tag at indexes 0 and 15. and a [Nan,Nan, END] tag at index 14. I need to make it for all my df. How could I do this?</p>