Clarified first-time install instructions

Just tried to clarify a few things that confused me as a newbie.  When I actually clicked on the link to the package manager documentation, it says that sometimes the packaged versions can be old.  I thought that information was helpful enough to belong here too (or instead?).

Many github projects are best installed by cloning the git repo.  I went down that path next and was left wondering why the 'master' branch wasn't working, so I clarified (in the link text) that you want to download the lein script from the *stable* branch.

Finally, the wording about the lein script bootstrapping itself and taking longer to run was all correct, but I didn't immediately connect the dots that "bootstrapping" meant that the only thing I had to do was to download the script from the right branch and run it.  So I changed "bootstrap" to "installs itself on the first run."  I think these are all extremely minor edits, but together, they might have set me on the simplest course of action more quickly.