PLEASE READ if you build the Ref Guide HTML locally

[Cassandra Targett <ct...@apache.org>]

If you build the Ref Guide HTML files on your local machine, please note
the following changes to the build.

To build the Ref Guide HTML version locally, you must have several Ruby
gems installed as pre-requisites. I've just committed changes that require
newer versions of these gems.

You can update your existing gems with the "gem update" command:

gem update asciidoctor
gem update jekyll-asciidoc
gem update slim
gem update tilt

And a new dependency needs to be added:

gem install concurrent-ruby

* The version of Jekyll has not changed so that gem does not need to be
updated.

The versions we're now using are documented in the Ref Guide README:
https://github.com/apache/lucene-solr/blob/master/solr/solr-ref-guide/README.adoc

See also https://issues.apache.org/jira/browse/SOLR-12786 for details of
these changes.

As soon as we move to Gradle, you won't need any gems locally. The PDF
build downloads a .jar, so while it is also updated no local changes are
needed (and it's going away soon anyway).

Let me know if you have trouble with the new versions -
Cassandra