Create README.md

Author: majecty

README.md

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+# Unity3dRichTextHelper
+This library helps you get substring of unity3d's rich  text.
+
+## How to use(simple)
+```csharp
+
+using RichTextSubstringHelper;
+
+var richText = "<color=blue>blue</color>black";
+richText.RichTextSubString(3); // <color=blue>blu</color>
+richText.RichTextSubString(6); // <color=blue>blue</color>bl
+```
+
+When you want to make a dialogue system, displaying richtext may cause a trouble.
+<image here>
+This library helps you get substring of richtext easily.
+
+## More example
+```csharp
+using System.Collections;
+using UnityEngine;
+using UnityEngine.UI;
+using RichTextSubstringHelper;
+
+public class NewBehaviourScript : MonoBehaviour {
+    [SerializeField]
+    Text uiText;
+    IEnumerator Start () {
+        var firstText = "<color=blue>blah</color>x";
+        for (int i = 0; i < firstText.RichTextLength(); i++)
+        {
+            yield return new WaitForSeconds(0.5f);
+            uiText.text = firstText.RichTextSubString(i + 1);
+        }
+    }
+
+}
+```
+
+## Performance
+
+`richText.RichTextSubString(i)` takes O(i) time because it should read string character by character every time.
+
+If you consider performance, use like below example.
+
+```csharp
+using System.Collections;
+using UnityEngine;
+using RichTextSubstringHelper;
+using UnityEngine.UI;
+
+public class NewBehaviourScript : MonoBehaviour {
+    [SerializeField]
+    Text uiText;
+
+    IEnumerator Start ()
+    {
+        var richText = "<color=blue>blue</color>black";
+        yield return null;
+        var maker = new RichTextSubStringMaker(richText);
+        
+        while (maker.IsConsumable())
+        {
+            maker.Consume();
+            uiText.text = maker.GetRichText();
+        }
+    }
+}
+
+```
+
+