Update README with information on where to obtain the zip distribution for Tomcat

[jajik]

In the README, there are several comments such as

Distribution archives are provided for each Tomcat version.

but I don't see any.

Does this still hold true? If yes, I would suggest clarifying (or even better adding links) , otherwise we should revise this.

[rhusar]

Yes, this is true - these are provided via maven, e.g. see https://repository.jboss.org/org/jboss/mod_cluster/mod_cluster-distribution/2.0.4.Final/

[rhusar]

Nevertheless - this should be better described better in the readme and the docs

[rhusar]

Also opened modcluster/docs.modcluster.io#190

[jajik]

Yes, that's what I meant. I haven't found any direct link. I'll try to propose something.