Add a chapter about r2js plugins

src/SUMMARY.md

@@ -119,6 +119,7 @@
    * [Analysis plugins](plugins/dev-anal.md)
    * [Bin plugins](plugins/dev-bin.md)
    * [Charset plugins](plugins/dev-charset.md)
+   * [R2JS plugins](plugins/r2js.md)
    * [Python plugins](plugins/python.md)
    * [Other plugins](plugins/dev-other.md)
    * [Troubleshooting](plugins/troubles.md)

src/plugins/r2js.md

@@ -0,0 +1,38 @@
+## R2JS Plugins
+
+The javascript runtime embedded in radare2 provides a way to implement different types of plugins.
+
+Check out the `r2skel` project for more examples, but we will cover the basics now.
+
+```js
+(function () {
+  r2.unload("core", "mycore");
+  r2.plugin("core", function () {
+    console.log("==> The 'mycore' plugin has been instantiated. Type 'mycore' to test it");
+    function coreCall(cmd) {
+      if (cmd.startsWith("mycore")) {
+        console.log("Hello From My Core!");
+        return true;
+      }
+      return false;
+    }
+    return {
+      "name": "mycore",
+      "license": "MIT",
+      "desc": "simple core plugin in typescript for radare2",
+      "call": coreCall,
+    };
+  });
+})();
+```
+
+Some notes on this code:
+
+* The whole code is wrapped inside an anonymous function call, this way we don't polute the global scope.
+* Use r2.unload() and r2.plugin() to unload and register new plugins
+* Register a plugin by passing the plugin type and a function
+* The initialization function returns a object describing it
+
+This code runs inside radare2, this means that it will be *fast*, and by fast I mean faster than Python, r2pipe and will be closer to the `C` plugins. Not just for running, but also for loading, because the js runtime is already there, use r2js plugins if possible if you care about performance.
+
+You can find other plugin examples in the examples directory in radare2.