added a node about HTML extension in readme
merged branch jfsimon/css-selector-readme-update (PR #8287)

This PR was submitted for the master branch but it was merged into the 2.3 branch instead (closes #8287).

Discussion
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[CssSelector] added a node about HTML extension in readme

It's a common mistake to use `CssSelector` with XML without knowing that `HTML` extension must be disbaled first (see #8286). This PR adds a note about that in the component's readme file.

Commits
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4235b26 [CssSelector] added a node about HTML extension in readme
[CssSelector] tweaked README file (closes #8287)