Readme: Fix optional paragraph

[cbachhuber]

• Added backticks around e.g. optional, such that the angle brackets are rendered in markdown
• Changed link for further info to the suitable chapter, as I did not find info on optional or templates in the internals chapter
• std::optional could be used as well, correct? Added to this paragraph

[codecov]

Codecov Report

Merging #357 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##           master   #357   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          12     12           
  Lines        3352   3352           
=====================================
  Hits         3352   3352

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[henryiii]

Thanks!