Link to changelog guildelines from contributing docs

[pquentin]

As noticed in #2393 where we had to ask for a changelog and it wasn't in the past tense.

[codecov]

Codecov Report

Merging #2415 (fba1cf8) into main (7c9ca88) will decrease coverage by 0.24%.
The diff coverage is n/a.

❗ Current head fba1cf8 differs from pull request most recent head 115b2f2. Consider uploading reports for the commit 115b2f2 to get more accurate results

@@             Coverage Diff             @@
##              main    #2415      +/-   ##
===========================================
- Coverage   100.00%   99.75%   -0.25%     
===========================================
  Files           25       25              
  Lines         2478     2478              
===========================================
- Hits          2478     2472       -6     
- Misses           0        6       +6     

Impacted Files	Coverage Δ
src/urllib3/util/wait.py	91.11% <0.00%> (-8.89%)	⬇️
src/urllib3/contrib/socks.py	97.29% <0.00%> (-2.71%)	⬇️
src/urllib3/response.py	99.75% <0.00%> (-0.25%)	⬇️

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[sethmlarson]

LGTM, thanks!