Add update instruction

changes/changelog.d/virtualenv.update

@@ -0,0 +1,2 @@
+With this update Funkwhale starts using poetry to maintain its dependencies. We therefore recommend
+removing the old virtualenv by running `rm -rf /srv/funkwhale/virtualenv`.

pyproject.toml

@@ -47,5 +47,10 @@
         name = "Removal"
         showcontent = true
 
+    [[tool.towncrier.type]]
+        directory = "update"
+        name = "Update instructions"
+        showcontent = true
+
 [tool.black]
     exclude = "(.git|.hg|.mypy_cache|.tox|.venv|_build|buck-out|build|dist|migrations|data)"