Update CONTRIBUTING.md

CONTRIBUTING.md

@@ -1,5 +1,7 @@
 # Submitting issues
 
+Please stop opening invalid issues regarding the "Legacy" label of uBlock Origin on AMO, it's all explained in the [Release notes](https://github.com/gorhill/uBlock/releases).
+
 For **support/discussions/help**, there is [/r/uBlockOrigin](https://www.reddit.com/r/uBlockOrigin/) on Reddit -- this is where I see the most activity for people helping each other regarding uBlock Origin.
 
 For **filter-related issues**, report on the respective filter list support site, or at [uBlockOrigin/uAssets](https://github.com/uBlockOrigin/uAssets/issues). Use [the logger](https://github.com/gorhill/uBlock/wiki/The-logger) to diagnose/confirm filter-related issues. If something does not work properly with uBO enabled, the **first step** is to rule out filter-related issues.