Add negative boolean expression note to CONTRIBUTING.md (#49033)
Add negative boolean expression note to CONTRIBUTING.md (#49033)

The team sometimes get questions around the use of `!foo` vs. `foo == false` in
PRs and reviews (e.g.  #48615). This change adds a bullet point to CONTRIBUTING.md
to make expectations here clearer and gives us something to point to in case of
discussion.
uschindler/elasticsearch,HonzaKral/elasticsearch,gingerwizard/elasticsearch,GlenRSmith/elasticsearch,scorpionvicky/elasticsearch,nknize/elasticsearch,HonzaKral/elasticsearch,GlenRSmith/elasticsearch,GlenRSmith/elasticsearch,nknize/elasticsearch,robin13/elasticsearch,HonzaKral/elasticsearch,nknize/elasticsearch,HonzaKral/elasticsearch,robin13/elasticsearch,gingerwizard/elasticsearch,scorpionvicky/elasticsearch,uschindler/elasticsearch,scorpionvicky/elasticsearch,gingerwizard/elasticsearch,robin13/elasticsearch,uschindler/elasticsearch,nknize/elasticsearch,scorpionvicky/elasticsearch,gingerwizard/elasticsearch,robin13/elasticsearch,gingerwizard/elasticsearch,GlenRSmith/elasticsearch,robin13/elasticsearch,gingerwizard/elasticsearch,gingerwizard/elasticsearch,uschindler/elasticsearch,GlenRSmith/elasticsearch,scorpionvicky/elasticsearch,uschindler/elasticsearch,nknize/elasticsearch