add correct mecab installation instructions

[stet-stet]

TL;DR
I present this PR to prevent people from having issues like #54, #18.

Update: I also strongly suspect that issues such as #111 was caused by an incorrect configuration of MeCab(with the default encodings), which may cause an assertion in fastBPE.hpp to fail (line 480), therefore resulting in failiure to produce output files after fastBPE.

At least in my system locale, failing to set any one of these utf-8-enabling flags(see install_external_tools.sh) led to empty outputs in the embed task, encoding errors (at $LASER/source/lib/romanize_lc.py), and much confusion. Regrettably, it is quite hard to know this fact before you have this problem.

Also, I changed README.md a bit, so that hopefully mecab feels a bit more optional for people not dealing with the Japanese language.

Additional question: Why was the auto-installation of Mecab dropped?

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[stet-stet]

I just noticed that #97 will install MeCab correctly, automatically. However since we do not know why the auto-installation of MeCab was dropped, I will still leave this PR open.

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