Add to use arc diff instead of launchpad

Summary: Use arc diff instead of launchpad for new contributions.

Test Plan: Proofread message

Reviewers: tsimonq2

Subscribers: lynorian

Differential Revision: https://phab.lubuntu.me/D42

CONTRIBUTING.md

@@ -77,10 +77,10 @@ git commit -m "I changed x in foo and y in bar" foo bar
 
 To submit your code, or push to your Launchpad account and
 submit a merge request, both will be seen by someone with the permission to
-merge. Push to the following repository to be able to submit a merge proposal
-in Launchpad:
-```
-git push git+ssh://LPUSERNAME@git.launchpad.net/~LPUSERNAME/lubuntu-manual/+git/ADDITION
+merge. Push to the following repository to be able to submit a merge proposal on differential after registering an account on the Lubuntu phabricator.
+
+``` 
+arc diff
 ```
 
 Another option if you prefer would be to send patch mail to [lubuntu-devel@lists.ubuntu.com](mailto:lubuntu-devel@lists.ubuntu.com).