Update README.md

Author: lexxmark

README.md

@@ -2,3 +2,85 @@ This library has some usefull Qt classes and widgets to show asyncronious operat
 
 [![qt-async demo video](http://img.youtube.com/vi/aTXOpmVRXq0/0.jpg)](https://youtu.be/aTXOpmVRXq0)
 
+# Basic use
+
+Declare async value object:
+```C++
+using AsyncQString = AsyncValue<QString>;
+AsyncQString value(AsyncInitByValue{}, "Hello World!");
+```
+
+When you need to calculate value, call one of the asyncXXX functions:
+```C++
+    asyncValueRunThreadPool(value, [](AsyncProgress& progress, AsyncQString& value) {
+
+        // a long calculations
+        for (auto i : {0, 1, 2, 3, 4})
+        {
+            // report progress
+            progress.setProgress(i, 5);
+            // check if calculation was stopped
+            if (progress.isStopRequested())
+            {
+                // report error
+                value.emplaceError("Stopped");
+                return;
+            }
+
+            QThread::sleep(1);
+        }
+
+        progress.setProgress(1.f);
+        progress.setMessage("Processing...");
+        QThread::sleep(1);
+
+        QString val = "Loaded value is 42";
+        // set calculated value
+        value.emplaceValue(std::move(val));
+
+    }, "Loading...", ASYNC_CAN_REQUEST_STOP::YES);
+```
+
+Somewhere in GUI code declare async widget:
+```C++
+        // create widget
+        auto valueWidget = new AsyncWidgetFn<AsyncQString>(ui->widget);
+        
+        // set callback that creates widget to show value
+        valueWidget->createValueWidget = [](QString& value, QWidget* parent) {
+            // create QLabel
+            return AsyncWidgetProxy::createLabel(value, parent);
+        };
+
+        // optionally set callback that creates progress widget
+        valueWidget->createProgressWidget = [this](AsyncProgress& progress, QWidget* parent)->QWidget* {
+            return new AsyncWidgetProgressSpinner(progress, parent);
+        };
+
+        // optionally set callback that creates error widget
+        valueWidget->createProgressWidget = [this](AsyncError& error, QWidget* parent)->QWidget* {
+            return new AsyncWidgetError(error, parent);
+        };
+
+        // assign value with widget
+        valueWidget->setValue(&m_value);
+```
+
+Instead of callbacks you can derive widget class from AsyncWidgetBase or AsyncWidget classes and override four functions:
+  
+```C++
+    // creates widget to show value
+    virtual QWidget* createValueWidgetImpl(ValueType& value, QWidget* parent) = 0;
+    
+    // creates widget to show error
+    virtual QWidget* createErrorWidgetImpl(ErrorType& error, QWidget* parent) = 0;
+    
+    // creates widget to show progress
+    virtual QWidget* createProgressWidgetImpl(ProgressType& progress, QWidget* parent) = 0;
+    
+    // create widget when no value has been assigned
+    virtual QWidget* createNoAsyncValueWidgetImpl(QWidget* parent) { return createLabel("<no value>", parent); }
+
+```
+
+# Rerunnable values