Merge pull request #3247 from DataDog/2.0-release-notes-etemplate

.github/PULL_REQUEST_TEMPLATE.md

@@ -5,6 +5,17 @@ for guidance on how to set up your development environment,
 run the test suite, write new integrations, and more.
 -->
 
+**2.0 Upgrade Guide notes**
+<!--
+(If this PR is for 1.x, please delete this section)
+A public facing description that will go into the [upgrade guide](https://github.com/DataDog/dd-trace-rb/blob/master/docs/UpgradeGuide.md) for this change.
+We already know what the PR does (from the PR title),
+but users should know either:
+* A migration path for this change. In other words, how to continue using their application without behavior changes
+in 2.0 (e.g. environment variable 'X' renamed to 'Y').
+* That there's no alternative; we completely dropped support for this feature.
+-->
+
 **What does this PR do?**
 <!-- A brief description of the change being made with this pull request. -->