Add java as an installation step

Guides/jenkins.md

@@ -4,6 +4,14 @@ How to install and administrate Jenkins.
 
 ## Install
 
+### Java
+
+You need Java 8 or 11 if you have Jenkins above 2.164.
+
+```
+sudo apt install openjdk-8-jre
+```
+
 ### Raspberry Pi
 
 [Jenkins doc : installation](https://jenkins.io/doc/book/installing/#debianubuntu)