Fix README to add instructions on how to clone properly with --recursive

Signed-off-by: garciadeblas <gerardo.garciadeblas@telefonica.com>

README.md

 # README
 
+## Cloning the repo
+
+You need to clone the repo with the option "--recursive" to fetch all the dependent git modules:
+
+```bash
+# Clone with SSH
+git clone --recursive ssh://git@osm.etsi.org:29419/vnf-onboarding/osm-packages.git
+# Clone with HTTPS
+git clone --recursive https://osm.etsi.org/gitlab/vnf-onboarding/osm-packages.git
+```
+
 ## Structure of the repo
 
 This repo contains OSM packages used for testing and for the hackfest.