document release contributors

```
% ~/code/incubator-pekko/scripts/authors.scala forked-from-akka v1.0.0-RC1
commits  added  removed
     53   3548     2157 PJ Fanning
     34   1514     1209 Matthew de Detrich
      3   2289     2053 Seeta Ramayya
      2    109      139 Sean Glover
      1    817     1378 Auto Format
```