docs(core): document timestamp format of Release struct

[MarcoIeni]

I copied this from here, which is the function git-cliff calls to populate that field.

[codecov-commenter]

Codecov Report

Merging #67 (cf3f59a) into main (9b5f0bb) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main      #67   +/-   ##
=======================================
  Coverage   40.44%   40.44%           
=======================================
  Files          13       13           
  Lines         831      831           
  Branches      218      218           
=======================================
  Hits          336      336           
  Misses        386      386           
  Partials      109      109           

Impacted Files	Coverage Δ
git-cliff-core/src/release.rs	14.29% <ø> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 9b5f0bb...cf3f59a. Read the comment docs.