Add documentation for nested selectors

[silvenon]

The documentation for nested selectors was missing, so I read the relevant tests and comments to figure out how it works exactly and added the documentation. I'm differentiating between three types of nested selectors:

1. regular selectors
2. class selectors from css
3. component selectors from styled

The first two require a leading &, but component selectors don't work that way, the & must be omitted. This inconsistency should probably be fixed at some point.

This is a part of #219.

[codecov-io]

Codecov Report

Merging #252 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #252   +/-   ##
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  Coverage   90.23%   90.23%           
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  Files          20       20           
  Lines         963      963           
  Branches      260      260           
=======================================
  Hits          869      869           
  Misses         76       76           
  Partials       18       18

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[tkh44]

I think it would but you would need the . before the interpolation.

[silvenon]

Ok, to your dot . commit I also added a sentence explicitly explaining why it's necessary in case someone misses it when reading the code snippet. I also squashed everything into a single commit, I hope that's a desired practice.