Simplify the README.rst file

[Gallaecio]

• Use bold face instead of italics for the first mention of the library name
• Move the license and language from the list of facts into the first paragraph
• Removed the list of supported Python versions, already indicated by batches, and not too important given that all non-EOL Python versions are supported
• Remove title case from “CSS3 Selectors” (→ CSS3 selectors)
• Move what XPath expressions allow to a separate paragraph, since that is not a core part of cssselect; cssselect use cases do not necessarily involve passing the result to an XML parser like lxml
• Remove the paragraph about cssselect being part of lxml in the past. It’s irrelevant for new users
• Moved the reference to the online documentation into a paragraph of its own, with the URL as display text

[codecov]

Codecov Report

Merging #103 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #103   +/-   ##
=======================================
  Coverage   95.49%   95.49%           
=======================================
  Files           2        2           
  Lines         754      754           
  Branches      128      128           
=======================================
  Hits          720      720           
  Misses         19       19           
  Partials       15       15           

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[codecov]

Codecov Report

Merging #103 (0dc1380) into master (79c341b) will not change coverage.
The diff coverage is n/a.

❗ Current head 0dc1380 differs from pull request most recent head 01eb4d1. Consider uploading reports for the commit 01eb4d1 to get more accurate results

@@           Coverage Diff           @@
##           master     #103   +/-   ##
=======================================
  Coverage   95.28%   95.28%           
=======================================
  Files           3        3           
  Lines         763      763           
  Branches      130      130           
=======================================
  Hits          727      727           
  Misses         20       20           
  Partials       16       16           

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Δ = absolute <relative> (impact), ø = not affected, ? = missing data
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[nyov]

Looks good