[DOC] Update CONTRIBUTING.md (#661)

Inform other contributors about the fact of starting to work on the issue

CONTRIBUTING.md

@@ -18,7 +18,8 @@ There are many ways to contribute:
 
 For all contributions, please respect the following guidelines:
 
-1. If you've noticed a bug or have a feature request, let us know in the [GitHub Issue tracker](https://github.com/process-analytics/bpmn-visualization-js/issues/new )! So we can confirm the bug or approve your feature, and provide feedback, before starting to code :slightly_smiling_face:
+1. If you've noticed a bug or have a feature request, let us know in the [GitHub Issue tracker](https://github.com/process-analytics/bpmn-visualization-js/issues/new )! So we can confirm the bug or approve your feature, and provide feedback, before starting to code :slightly_smiling_face:  
+If you start working on one of the existing [issues](https://github.com/process-analytics/bpmn-visualization-js/issues), please leave a comment below the issue so everyone knows that the issue is taken.
 
 2. Do the changes in your own [fork](CONTRIBUTING.md#fork--create-a-branch) of the code
 
@@ -44,7 +45,9 @@ test everything)
 
 [Fork bpmn-visualization](https://help.github.com/articles/fork-a-repo) and create a branch with a descriptive name. 
 
-A good branch name would be (where issue #25 is the ticket you're working on): **25-annotations_to_tasks**
+A good branch name would be (where issue #25 is the ticket you're working on): **25-annotations_to_tasks**  
+
+:information_source: please don't forget to leave a comment under the issue to let other contributors know that this issue is under the development \[i.e "in progress", "i am on it"\]
 
 ```sh
 git checkout -b 25-annotations_to_tasks