Add syntax highlighting in README

[adamchainz]

Description

title says it all

Checklist

• [n/a] I have ran tox to ensure tests pass
• [n/a] Usage documentation added in case of new features
• [n/a] Tests added / I have not lowered coverage from 100%

[codecov]

Codecov Report

Merging #38 (52af3fa) into master (3bd6255) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##            master       #38   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            3         3           
  Lines           87        87           
  Branches        17        17           
=========================================
  Hits            87        87           

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[marksweb]

Ah yes, nice touch. Thanks @adamchainz!