docs(readme): add APIdia badge

Author: spannm

README.md

@@ -6,6 +6,7 @@
   <a href="https://github.com/spannm/ucanaccess/actions/workflows/ci_jdk11_ubuntu.yml"><img src="https://img.shields.io/github/actions/workflow/status/spannm/ucanaccess/ci_jdk11_ubuntu.yml?label=Build%20(JDK%2011%20Linux)&style=flat-square" alt="GitHub Actions Workflow Status"></a>
   <a href="https://github.com/spannm/ucanaccess/actions/workflows/ci_jdk11_win.yml"><img src="https://img.shields.io/github/actions/workflow/status/spannm/ucanaccess/ci_jdk11_win.yml?label=Build%20(JDK%2011%20Win)&style=flat-square" alt="GitHub Actions Workflow Status"></a>
   <a href="https://javadoc.io/doc/io.github.spannm/ucanaccess"><img src="https://javadoc.io/badge2/io.github.spannm/ucanaccess/javadoc.svg?style=flat-square" alt="Javadoc"></a>
+  <a href="https://apidia.net/mvn/io.github.spannm/ucanaccess"><img src="https://apidia.net/mvn/io.github.spannm/ucanaccess/badge_flat_square.svg" alt="APIdia"></a>
 </div>
 
 <h1 align="center">UCanAccess</h1>