Update PULL_REQUEST_TEMPLATE.md

Author: arcanis

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,13 +1,30 @@
-<!-- Thanks for submitting a pull request! Please provide enough information so that others can review your pull request. The two fields below are mandatory. -->
+<!-- 
 
-**Summary**
+  Important note: This repository is about Yarn 1.x, also called "Classic". We now release
+  modern versions on the yarnpkg/berry repository, which has a completely new (and more
+  stable) codebase.
+  
+  If you get a problem with Yarn 1.x, it is *very* likely to have been fixed in recent
+  versions. We recommand that you migrate when you get the chance: the process has been
+  refined over the years and should be mostly painless - check here for details:
+  
+    https://yarnpkg.com/getting-started/migration#why-should-you-migrate
+  
+  If you hit blockers that aren't addressed in this guide, feel free to ask for help on
+  our Discord community server, or our GitHub discussion forum:
+  
+    https://discord.com/invite/yarnpkg
+    https://github.com/yarnpkg/berry/discussions
 
-<!-- Is the feature a substantial feature request? Please use https://github.com/yarnpkg/rfcs -->
+  Finally, if you intend to open a bug on modern versions of Yarn, the right tracker is here:
+  
+    https://github.com/yarnpkg/berry
 
-<!-- Explain the **motivation** for making this change. What existing problem does the pull request solve? -->
+  If you decide to stay on Yarn 1 regardless, please be aware that we don't plan to
+  merge pull requests or release future versions of Classic unless it's to solve an
+  critical vulnerability report (which is unlikely). The modern releases are
+  however very actively supported, and your help would be appreciated.
 
-<!-- Don't forget to update the CHANGELOG.md to quickly describe your changes to other users! -->
+  Thanks for your understanding!
 
-**Test plan**
-
-<!-- Demonstrate the code is solid. Example: The exact commands you ran and their output, screenshots / videos if the pull request changes UI. -->
+-->