README.md updates for Mac Installs

[UncleTallest]

added Mac OS X specific installation instructions to the top-level README.md file.

[bbc-jenkins]

Can one of the admins verify this patch?

• \"ok to test\" to accept this pull request for testing
• \"test this please\" for a one time test run
• \"add to whitelist\" to add the author to the whitelist

[bbc-jenkins]

Can one of the admins verify this patch?

• \"ok to test\" to accept this pull request for testing
• \"test this please\" for a one time test run
• \"add to whitelist\" to add the author to the whitelist