docs: add photoprism to gin's user list

[thinkerou]

Asked @lastzero for and got permission to feature this project at https://github.com/photoprism/photoprism

Them API server: https://github.com/photoprism/photoprism/tree/develop/internal

[codecov]

Codecov Report

Merging #1601 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1601   +/-   ##
=======================================
  Coverage   99.27%   99.27%           
=======================================
  Files          39       39           
  Lines        1928     1928           
=======================================
  Hits         1914     1914           
  Misses         10       10           
  Partials        4        4

---

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 333bac5...9031256. Read the comment docs.