plasmapper install doc: remind to install java if needed

doc/install_plasmapper.md

 # Instructions to install plasMapper on your server 
 
-On the targeted server type the following commands in a terminal :
+An installation step of plasmapper requires that you have java installed on your server (command jar). If this is not the case yet, type the following command (for Ubuntu):
+```sh
+sudo apt install default-jdk
+```
+Then, to install plasmapper on your server, type the following commands in a terminal :
 ```sh
    # extract PlasMapper source code
    cd opts/