Issue template: warn about known unstability

ISSUE_TEMPLATE

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-**Operating system or device - Godot version:**
-
-
-**Issue description** (what happened, and what was expected):
-
-
-**Steps to reproduce:**
-
-
-**Link to minimal example project** (optional but very welcome):

ISSUE_TEMPLATE.md

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+vvv Remove me vvv
+
+*NOTE:* If you using the current master branch / 3.0-alpha version, do not that
+breakage is *expected*. Projects from Godot 2.x are expected not to work. Please
+wait for the upcoming stabilisation period to report bugs regarding recent changes.
+
+^^^ Remove me ^^^
+
+
+**Operating system or device - Godot version:**
+
+
+**Issue description** (what happened, and what was expected):
+
+
+**Steps to reproduce:**
+
+
+**Link to minimal example project** (optional but very welcome):