Update changelog.

CHANGELOG

@@ -50,6 +50,7 @@ v 7.8.0 (unreleased)
   - Prevent losing unsaved comments by automatically restoring them when comment page is loaded again.
   - Don't allow page to be scaled on mobile.
   - Clean the username acquired from OAuth/LDAP so it doesn't fail username validation and block signing up.
+  - Show users button to share their newly created public or internal projects on twitter
 
 v 7.7.2
   - Update GitLab Shell to version 2.4.2 that fixes a bug when developers can push to protected branch