[release] Updates release doc: creates branch stable/release by using frama-ci.

doc/release/branch.tex

@@ -22,8 +22,19 @@ $\rightarrow$ Repository $\rightarrow$ Protected Branches.
 \section{Creating the branch}
 
 Create the branch \texttt{stable/release} where \texttt{release} is the
-element name.
-
+element name using \texttt{frama-ci-create-branch} tool:
+\begin{enumerate}
+\item Install \texttt{frama-ci} tools:
+\begin{shell}
+opam pin add frama-ci-tools git@git.frama-c.com:frama-c/Frama-CI.git
+\end{shell}
+\item Create an API token for gitlab, in your gitlab profile settings
+\item Run the command
+\begin{shell}
+frama-ci-create-branch --token=\$TOKEN \
+--new-branch=stable/release --default-branch
+\end{shell}
+\end{enumerate}
 What can be committed in this branch must follow the release schedule,
 and go through Merge-requests. Everything else should go in \texttt{master},
 which can then be reset to standard-level protection (Developers + Masters