README: Describe usage of common-subtree.sh

Signed-off-by: Frédéric Dalleau <frederic.dalleau@collabora.co.uk>

README.md

@@ -2,4 +2,28 @@ Common files shared accross apertis tests.
 
 Those files are shared through a git subtree which is included by all test projects.
 
-Use common-subtree.sh to add the subtree in a tests repository
+Use `common-subtree.sh add` to add the subtree in a tests repository.
+For example:
+```
+# Retrieve the common-subtree.sh script, we use `git`, but could also use `wget`
+$ git clone git@gitlab.apertis.org:tests/common
+
+# Clone the test repository if needed
+$ git clone git@gitlab.apertis.org:tests/iptables-basic
+# Enter the test directory
+$ cd iptables-basic
+
+# In the test directory, add the common git subtree
+$ ../common/common-subtree.sh add
+
+# A new commit has been created, push the test with the subtree initialized 
+$ git push origin
+```
+
+If a script is supposed to be in the common/ directory, it should be commited in the
+[common repository](https://gitlab.apertis.org/tests/common), and then it can be pulled
+into the git subtree using the command:
+```
+$ ../common/common-subtree.sh pull
+
+```