doc: add releases

Signed-off-by: Iker Pedrosa <ipedrosa@redhat.com>

doc/contributions/releases.md

@@ -0,0 +1,7 @@
+# Releases
+
+The shadow project doesn't follow any specific timeline to release new software
+versions. Usually, they are released when a major milestone is finished.
+
+Released source code, alongside the release notes, are provided in the
+[release Github page](https://github.com/shadow-maint/shadow/releases).