Fix typos in read me, remove redundancy in compose file, and add Plugins.txt

[zanetworker]

Fix typos in Readme:

• Ports were set to 8080 instead of 8081 in dummy Jenkins container creation
• Password was document as "password" instead "admin" was actually used

Redundancy in compose file:

• replace data-containers with normal persistent volumes (data-containers add an extra level of dependancy and can cause glitches with containers which are later pruned)
• remove directories for data-containers

Add Plugin.txt:

• it can be useful to add a default plugin.txt for users who are not willing to create a dummy container only for the sake of getting the default plugins.

[michaellihs]

Hi @zanetworker - thanks for the feedback - with two sick women at home, I didn't have the time to come back to this any earlier :)

I have two questions:

1. Will this volumes configuration in the compose file create a mounted volume on the host and mount it to the container (without any further tasks to be run on the host):

   volumes:
     jenkinsdata:
     jenkinsconf:
   
   #...
   
   services:
     jenkinsmaster:
       volumes:
         - jenkinsdata:/var/log/jenkins
         - jenkinsconf:/var/jenkins_home

2. Did you see the jenkins-master/plugins.txt, since I think this is the same file as you created.

Besides, I "cherry picked" your changes manually - thanks a lot for your effort!