Add postcss-class-name-shortener to docs/plugins.md (#1199)
Add postcss-class-name-shortener to docs/plugins.md (#1199)

* Add postcss-class-name-shortener to Optimizations

* Reposition into alphabetical order

postcss/postcss,jonathantneal/postcss,bezoerb/postcss,Semigradsky/postcss,postcss/postcss