Amend documentation to install globally

[slavaaaaaaaaaa]

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I believe the intention here is to install this package globally so we can use it across all projects. --save wasn't working for me at all, while -g installed it into /usr/local/bin/ and instantly provided me with a working binary.

Thank you for your work!

[skywinder]

+1

[TT--]

related, https://github.com/jonschlinkert/markdown-toc/pull/96

[jhannah]

Yes, merge this please, would have saved me 15m today. :) Thanks!

[refs]

+1, I found myself doing the same, glad someone else opened a PR :)