Add --model_dir flag as the training code requires this field to be set.

[mohitreddy1996]

Set model_dir flag in README as the training code expects the flag to be set. Tensorboard summary writer otherwise fails with

"TypeError: Expected binary or unicode string, got None"

[codecov-commenter]

Codecov Report

Merging #281 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #281   +/-   ##
=======================================
  Coverage   79.39%   79.39%           
=======================================
  Files          34       34           
  Lines        2252     2252           
=======================================
  Hits         1788     1788           
  Misses        464      464           

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[avital]

Can we also make model_dir no longer have a default, or force it to have a value with flags.flags.mark_flag_as_required('filename')? The error message you got if you didn't set the model_dir flag isn't good.