Update README.markdown

I updated README.markdown for the current system requirements and removed references to the "alcor" branch as it has not been updated in two years. Also, Quicksilver is no longer a beta. ;-)
Update README.markdown

Updated README.markdown to incorporate suggestions by @skurfer: removed an obsolete warning, extended note on issues with multiple Quicksilver builds installed to mention AppleScript.
Merge pull request #1684 from RobFog/master

Update README.markdown