suggest "xcode-select --install" for OS X 10.9; soft link to julia in /usr/local/bin not enough

- highlight that soft link to julia executable in /usr/local/bin is not enough (at least for me on OS X 10.9; it creates some obscure error message related to OPEN_BLAS and quits julia)
- in the OS X section, suggest running "xcode-select --install" for  10.9 Mavericks
Xcode command line utils; getting julia in your path
Merge pull request #4781 from fabianlischka/patch-1

suggest "xcode-select --install" for OS X 10.9; soft link to julia in /u...