message: Add -l for model argument for main.py gtp examples.
  Also add BOARD_SIZE=19.
diff: @@ -186,7 +186,7 @@ and you can use any gtp-compliant program with it.
  ```
  # Latest model should look like: /path/to/models/000123-something
  LATEST_MODEL=$(ls -d $MINIGO_MODELS/* | tail -1 | cut -f 1 -d '.')
  -python3 main.py gtp $LATEST_MODEL -r $READOUTS -v 3
  +BOARD_SIZE=19 python3 main.py gtp -l $LATEST_MODEL -r $READOUTS -v 3
  ```
  (If no model is provided, it will initialize one with random values)
  @@ -207,13 +207,13 @@ speaks GTP.) You can download the gogui set of tools at
  GTP](http://gogui.sourceforge.net/doc/reference-twogtp.html).
  ```shell
  -gogui-twogtp -black 'python3 main.py gtp gs://$BUCKET_NAME/models/000000-bootstrap' -white 'gogui-display' -size 19 -komi 7.5 -verbose -auto
  +gogui-twogtp -black 'python3 main.py gtp -l gs://$BUCKET_NAME/models/000000-bootstrap' -white 'gogui-display' -size 19 -komi 7.5 -verbose -auto
  ```
  Another way to play via GTP is to watch it play against GnuGo, while spectating the games
  ```
  BLACK="gnugo --mode gtp"
  -WHITE="python3 main.py gtp path/to/model"
  +WHITE="python3 main.py gtp -l path/to/model"
  TWOGTP="gogui-twogtp -black \"$BLACK\" -white \"$WHITE\" -games 10 \
  -size 19 -alternate -sgffile gnugo"
  gogui -size 19 -program "$TWOGTP" -computer-both -auto
  