Update README.md

Author: 1001binary

README.md

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 # Recaptcha-mvc
 The .NET library for integrating Google Recaptcha V3 into ASP.NET MVC 3 or higher.
 
+
+
 ### Usage
 
 - Suppose you have a ASP.NET MVC project.
@@ -9,6 +11,36 @@ The .NET library for integrating Google Recaptcha V3 into ASP.NET MVC 3 or highe
 - Download the sample project from this repository.
 - Follow it to integrate Google Recaptcha V3 into your web application.
 
+### API
+
+#### HtmlHelper static class
+- **HtmlHelper.SetupHiddenRecaptcha**: insert the recaptcha hidden field.
+- **HtmlHelper.SetupClientScript**: insert the Google Recaptcha v3 script and lib's script to display captcha.
+
+#### CommonHelper static class
+- **CommonHelper.ValidateRecaptchaV3**: validate the captcha against secret key and token.
+- **CommonHelper.AddErrorAndRedirectToGetAction**: add error to TempData and redirects to the current action.
+- **CommonHelper.RedirectToUrl**: redirects to a specific action.
+- **CommonHelper.AddErrorToModelState**(by default): adds an error message to ModelState in the current action.
+
+#### Recaptcha3ActionFilterAttribute filter
+
+This filter is used for verifying the captcha for a number of required URLs. This accepts three parameters:
+
+- SecretKey: the secret key of Google Recaptcha V3 service.
+- FilterItems: a number of conditions which are valid for the catpcha verification.
+- OnFailure: in case the catpcha verification is failed, this action will be executed. By default, **CommonHelper.AddErrorToModelState**
+- RecaptchaFieldName: the HTML field name of the captcha. By default, **recaptcha_response**.
+
+### Feedback
+Any feedback are appreciated a lot.
+
+### Change Logs
+
+#### v0.1.0
+
+- Supports ASP.NET MVC 3 and higher.
+
 ### Copyright and License
 ©Copyright 2020 by 1001binary
 MIT License