calc() description missing in the README

[renestalder]

In the table of contents, calc() is linked, but there is no chapter about calc in the readme.

[MadLittleMods]

This section was added in 3da59e5#diff-04c6e90faac2675aa89e2176d2eec7d8R281 and removed in 5b2363f#diff-04c6e90faac2675aa89e2176d2eec7d8L281. Some discussion here https://github.com/MadLittleMods/postcss-css-variables/pull/36/files#r69385635

@renestalder Mind creating a PR to remove it from the table of contents?