title: Fillna of one column with random float values in range in Pandas
body: <p>I have such dataframe, how can I <code>fillna</code> with random float values from <code>-0.5</code> to <code>0.5</code> (bounds included) for <code>value</code> column?</p>
  
  <pre><code>  city district       date  value
  0    a        b   2019/8/1   0.15
  1    a        b   2019/9/1   0.12
  2    a        b  2019/10/1    NaN
  3    c        d   2019/8/1   0.03
  4    c        d   2019/9/1  -0.36
  5    c        d  2019/10/1    NaN
  </code></pre>
  
  <p>Thank you.</p>