Update README.md

CentOS 5 hints work for RHEL 5 as well
Add warning for missing module in myid() == 1
fix new writemime() for arrays
Merge pull request #5851 from JuliaLang/teh/deserialize

Add warning for missing module in myid() == 1
Merge branch 'master' of github.com:JuliaLang/julia into jb/arrayrepr