Added page with description JS interop

Author: trueromanus

docs/jsinterop.md

@@ -0,0 +1,64 @@
+## JavaScript Interop
+
+Usually, to interact with JavaScript, we need to either execute some code from C# on the JavaScript side or get value(s) from the JavaScript side in C#.
+
+### How to execute JavaScript code from C#?
+
+First of all, any JavaScript code performed inside window scope, which mean we need to know window.  
+After it we can write code as follow:
+```csharp
+// our window pointer
+var windowPointer = ...;
+// script that will be performed in window scope
+var script = '2 + 3';
+
+SciterValue result;
+
+// evaluate
+Host.ExecuteWindowEval(windowPointer, script, out result);
+
+var calculatedValue = Host.GetValueInt32(ref result);
+
+Console.WriteLine("Result is " + calculatedValue);
+```
+
+### How to execute C# code from JavaScript?
+
+With EventHandler we can handle a lot of changes to an element, but what if we want to literally call C# methods from JavaScript?
+We can do this, but it will look like we are handling an event from the element.  
+
+First of all we need to markup as follow:
+```html
+...
+<!-- we define some element in HTML and attach our behaviour -->
+<div id="myid" style="behavior: mybehaviour"></div>
+...
+<script>
+const element = document.getElementById('myid');
+const result = element.xcall("csharpmethod", "string1", 1);
+console.log(result);
+</script>
+```
+
+Second we need to handle call in C# and to do something:
+```csharp
+public class MyHandler : ElementEventHandler {
+
+    public MyHandler ( nint element, SciterAPIHost host ) : base ( element, host ) {}
+
+    public override (SciterValue? value, bool handled) ScriptMethodCall ( string name, IEnumerable<SciterValue> arguments ) {
+	// only if called csharpmethod
+        if ( name == "csharpmethod" ) {
+                // read first string parameter
+		var stringParameter = arguments.ElementAt ( 0 );
+		var stringValue = Host.GetValueString(ref stringParameter);
+                // read second number parameter
+		var intParameter = arguments.ElementAt ( 1 );
+		var intValue = Host.GetValueInt32(ref intParameter);
+                // create result value
+		return (Host.CreateValue(stringValue + intValue), true);
+	}
+
+        return (null, false);
+    }
+```

mkdocs.yml

@@ -10,4 +10,5 @@ nav:
   - Adding Event handlers: eventhandlers.md
   - Register file protocols: fileprotocols.md
   - Find element and change content: fileelement.md
-  - Using Values: createvalue.md
+  - Using Values: createvalue.md
+  - JavaScript Interop: jsinterop.md