Add instructions for keeping in sync with upstream repo

Signed-off-by: Michel Alexandre Salim <michel@michel-slm.name>

README.md

@@ -1,3 +1,16 @@
 # openssl3
 
 The openssl3 package
+
+This is lightly forked from, and should be kept in sync with,
+CentOS Stream 9's [openssl](https://gitlab.com/redhat/centos-stream/rpms/openssl).
+
+Add this as a remote to ease merging changes, e.g.:
+
+    git remote add gitlab https://gitlab.com/redhat/centos-stream/rpms/openssl.git
+
+And merge changes
+
+    git fetch gitlab
+    git merge gitlab/c9s
+