Updates to the documentation instructions for downloading Mesos

[Andy Konwinski <an...@berkeley.edu>]

Thanks to Ben's recent efforts to get the SVN repo up to date with github,
and in line with his encouragement to start using SVN/git-svn for
development going forward, I just updated the documentation at
https://github.com/mesos/mesos/wiki for how to download Mesos. The updated
section now reads:
Downloading Mesos

We are actively working on our first official Apache release. Currently,
you can obtain Mesos (the current development HEAD) by checking it out from
either the Apache SVN or Apache Git repository (the git repo is a mirror of
the SVN repo)

   - For SVN, use: svn co
   https://svn.apache.org/repos/asf/incubator/mesos/trunk mesos-trunk
   - For git, use: git clone git://git.apache.org/mesos.git

NOTES: The Apache SVN repository is the definitive spot for the source code
now. For alpha version 0.4 and before, you can still download tagged “alpha
releases” from github via the tags page<https://github.com/mesos/mesos/tags>.
However, we do not recommend using the Github repository hosted under the
mesos user (e.g. git://github.com/mesos/mesos.git) any longer. Though it is
a little confusing, Apache also maintains a github clone of the SVNrepository
at https://github.com/apache/mesos, which should be fine to use as an
alternative to the SVN and git apache.org repositories listed above (though
the committers don’t use it).

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What do people think of the new wording?

Also, we could consider making the github.com/mesos/mesos repo private to
minimize the confusion (which the big NOTE above tries to explain to folks)
and avoid us having to figure out a strategy to keep it syned with the
apache repos. However, if we did that we would lose the history and the
tags (we lost the tags in the move to SVN and I don't think we can
re-create an equivalent svn tag to the github/mesos/mesos alpha 0.4 tag
either).

I'm super excited to be moving more towards the Apache way, and I know Ben
and the rest of the team is too!

Andy