Update README.md (DataDog#23323)

Adds the "why" to encourage internal contributors to follow branch naming convention.

Author: rtrieu

README.md

@@ -17,7 +17,7 @@ For more information on contributing, see the [contribution guidelines][18].
 ### Datadog Staff
 
 - Always branch off of master; never commit directly to master.
-- Name your branch `<SLACK_HANDLE>/<FEATURE_NAME>`.
+- Name your branch `<SLACK_HANDLE>/<FEATURE_NAME>`. If you don’t use this convention, the Gitlab pipeline won’t run and you won’t get a branch preview. Getting a branch preview makes it easier for us to check for any issues with your PR, such as broken links.
 - Consult our [contributing guidelines][8].
 - When you're ready to commit, create a new pull request to master from your branch.
 - Use GitHub's [draft pull request][15] feature and appropriate labels such as "Do Not Merge" or "Work in Progress" until your PR is ready to be merged and live on production.