Added slack notification

- Also fix mistyped badge url

.github/workflows/gradle.yml

@@ -45,3 +45,12 @@ jobs:
                     name: codecov-umbrella # optional
                     fail_ci_if_error: true # optional (default = false)
                     verbose: true # optional (default = false)
+            -   name: action-slack
+                uses: 8398a7/action-slack@v3
+                with:
+                    status: ${{ job.status }}
+                    author_name: github_worker
+                    fields: repo,message,commit,author,action,eventName,ref,workflow,job,took
+                    if_mention: failure,cancelled
+                env:
+                    SLACK_WEBHOOK_URL: ${{ secrets.SLACK_WEBHOOK_URL }} # required

README.md

@@ -2,7 +2,7 @@
 
 [![MIT License](https://img.shields.io/badge/license-MIT-blue.svg)](https://opensource.org/licenses/MIT)
 [![LoC](https://tokei.rs/b1/github/aergoio/heraj)](https://github.com/aergoio/heraj)
-[![github build](https://github.com/aergoio/aergo/actions/workflows/gradle.yml/badge.svg)](https://github.com/aergoio/aergo/actions/workflows/gradle.yml/badge.svg)
+[![github build](https://github.com/aergoio/heraj/actions/workflows/gradle.yml/badge.svg)](https://github.com/aergoio/aergo/actions/workflows/gradle.yml/badge.svg)
 [![codecov.io](http://codecov.io/github/aergoio/heraj/coverage.svg?branch=develop)](http://codecov.io/github/aergoio/heraj?branch=develop)
 
 The hera is the client-side framework for the aergo.