add issue template,

not to create issues about latest images

.github/issue_template.md

+---
+name: Tracking issue
+about: This repository is the source for R <= 3.6.3 images built before 2020-04; issues for R >= 4.0.0 images built after 2020-04 should be created in https://github.com/rocker-org/rocker-versioned2, not here.
+---
+
+<!-- This repository is the source for R <= 3.6.3 images built before 2020-04; issues for R >= 4.0.0 images built after 2020-04 should be created in https://github.com/rocker-org/rocker-versioned2, not here. -->