Enhancement: Use Markdown for license

[localheinz]

This PR

• uses Markdown for the license

[codecov]

Codecov Report

Merging #37 into master will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master      #37   +/-   ##
=========================================
  Coverage     98.73%   98.73%           
  Complexity       53       53           
=========================================
  Files            13       13           
  Lines           158      158           
=========================================
  Hits            156      156           
  Misses            2        2

Impacted Files	Coverage Δ	Complexity Δ
src/Year.php	100% <ø> (ø)	6 <0> (ø)	⬇️
src/Exception/InvalidRange.php	100% <ø> (ø)	2 <0> (ø)	⬇️
src/Template.php	93.33% <ø> (ø)	11 <0> (ø)	⬇️
src/File.php	100% <ø> (ø)	5 <0> (ø)	⬇️
src/Url.php	100% <ø> (ø)	4 <0> (ø)	⬇️
src/Exception/InvalidReplacements.php	100% <ø> (ø)	2 <0> (ø)	⬇️
src/Exception/InvalidFile.php	100% <ø> (ø)	3 <0> (ø)	⬇️
src/Exception/InvalidUrl.php	100% <ø> (ø)	1 <0> (ø)	⬇️
src/Holder.php	100% <ø> (ø)	5 <0> (ø)	⬇️
src/Exception/InvalidYear.php	100% <ø> (ø)	1 <0> (ø)	⬇️
... and 3 more

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