doc: Add srilm_root to .speechrc in README.md

[mpuels]

The script speech_build_lm.py requires the configuration variable srilm_root. Otherwise, running it yields

Traceback (most recent call last):
  File "./speech_build_lm.py", line 81, in <module>
    srilm_root       = config.get("speech", "srilm_root")
  File "/home/ubuntu/anaconda3/envs/tensorflow_p27/lib/python2.7/ConfigParser.py", line 618, in get
    raise NoOptionError(option, section)
ConfigParser.NoOptionError: No option 'srilm_root' in section: 'speech'

[gooofy]

very cool, thank you! :)