chore: Add signed commits requirement to CONTRIBUTING.md

[ncreated]

What does this PR do?

📦 Adding a requirement for signing commits to CONTRIBUTING.md.

Counterpart to DataDog/dd-sdk-ios#1709

Motivation

To be specific in what we require for external contributions.

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• Make sure you discussed the feature or bugfix with the maintaining team in an Issue
• Make sure each commit and the PR mention the Issue number (cf the CONTRIBUTING doc)

[codecov-commenter]

Codecov Report

Merging #1905 (ce92ac7) into develop (87f81fe) will decrease coverage by 0.03%.
The diff coverage is n/a.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1905      +/-   ##
===========================================
- Coverage    83.42%   83.39%   -0.03%     
===========================================
  Files          477      477              
  Lines        16922    16915       -7     
  Branches      2514     2514              
===========================================
- Hits         14116    14105      -11     
- Misses        2119     2124       +5     
+ Partials       687      686       -1     

see 28 files with indirect coverage changes