document releasing process in dev doc

Author: MaxThevenet

Docs/source/developers/developers.rst

@@ -20,3 +20,4 @@ Our Doxygen API documentation in classic formatting `is located here <../_static
    portability
    profiling
    documentation
+   workflow

Docs/source/developers/workflow.rst

@@ -0,0 +1,19 @@
+.. _developers-workflow:
+
+Workflow
+========
+
+Make a new Github release
+-------------------------
+
+WarpX has one release per month. To make the release, you need to:
+
+    * Update the version number in all source files. There is a script for that, so you can do:
+
+      .. code-block:: sh
+          cd Tools/
+          ./update_release.sh
+
+    * Merge ``dev`` branch into ``master`` branch.
+
+    * Click the `Draft a new release` button at https://github.com/ECP-WarpX/WarpX/releases and follow instructions.