Please avoid pushing commits to the emacs-26 branch

[Eli Zaretskii]

Since RC1 of Emacs 26.3 is out, please don't push commits to the
emacs-26 branch until further notice.  If no significant problems are
reported with the RC in a couple of weeks, we will declare that RC1 is
the official 26.3 release, and after that the branch will be once
again open for changes according to our usual policies (documentation
and safe changes; see CONTRIBUTE).

Thanks.