Added some usage notes to the README

[bitstorm]

Hi,

I've started to look at your library as a replacement for jsass. I think it might be helpful adding some usage notes to the README as a basic reference for those who want to adopt your library. For me the most prominent scenario is customizing Bootstrap through SASS files, so I've added a section dedicated to this topic.

Hope this could help the project.

BTW nice job! :-)

[codecov]

Codecov Report

Merging #131 (e79756f) into master (aba4f66) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #131   +/-   ##
=========================================
  Coverage     65.32%   65.32%           
  Complexity      243      243           
=========================================
  Files            39       39           
  Lines          1015     1015           
  Branches        167      167           
=========================================
  Hits            663      663           
  Misses          263      263           
  Partials         89       89           

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more

[larsgrefer]

Thanks for your contribution