doc: contribute: adding supportive jpg image

Add the jpg image used in the documentation for
Project Roles.

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>
diff --git a/doc/contribute/img/img_release_activity.png b/doc/contribute/img/img_release_activity.png
new file mode 100644
index 0000000..c15429b
--- /dev/null
+++ b/doc/contribute/img/img_release_activity.png
Binary files differ