Created Mixin support (markdown)

Mixin-support.md

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+# Base set-up for Minestom extensions
+In your extension.json file, add this line:
+```
+  "mixinConfig": "mixins.testextension.json"
+```
+Replace `mixins.testextension.json` with the name of the JSON configuration file for Mixin.
+
+Example of Mixin configuration for Minestom (`mixins.testextension.json`):
+```
+{
+  "required": true,
+  "minVersion": "0.8",
+  "package": "testextension.mixins",
+  "target": "@env(DEFAULT)",
+  "compatibilityLevel": "JAVA_11",
+  "mixins": [
+    "InstanceContainerMixin",
+    "DynamicChunkMixin"
+  ]
+}
+```
+
+For more information, check the Mixin wiki and documentation: [Wiki](https://github.com/SpongePowered/Mixin/wiki) and [Documentation](https://docs.spongepowered.org/stable/fr/contributing/implementation/mixins.html)