contrib : add notes about merging PRs (#15881)
* contrib : add notes about merging PRs
* Update CONTRIBUTING.md
Co-authored-by: Diego Devesa <slarengh@gmail.com>
* Update CONTRIBUTING.md
Co-authored-by: Johannes Gäßler <johannesg@5d6.de>
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Co-authored-by: Diego Devesa <slarengh@gmail.com>
Co-authored-by: Johannes Gäßler <johannesg@5d6.de>