Add Tuleap to the list of tools supporting to send a push notification

[LeSuisse]

Add Tuleap to the list of tools supporting to send a push notification

The main point is to help Tuleap users to find their ways when they setup the Git plugin with Tuleap.

Checklist

Put an x in the boxes that apply. You can also fill these out after creating the PR. If you're unsure about any of them, don't hesitate to ask. This is simply a reminder of what we are going to look for before merging your code. If a checkbox or line does not apply to this pull request, delete it. We prefer all checkboxes to be checked before a pull request is merged

• I have read the CONTRIBUTING doc
• I have referenced the Jira issue related to my changes in one or more commit messages
• I have added tests that verify my changes
• Unit tests pass locally with my changes
• I have added documentation as necessary
• No Javadoc warnings were introduced with my changes
• No spotbugs warnings were introduced with my changes
• Documentation in README has been updated as necessary
• Online help has been added and reviewed for any new or modified fields
• I have interactively tested my changes
• Any dependent changes have been merged and published in upstream modules (like git-client-plugin)

As this is only a change in the README, the checklist is incomplete.