Clarify Mac OS X experience. Signed-off by: Mary Anthony (moxieandmore@g...

...mail.com)

Newb here just running through the process for setting up dev environment and contributing.  In the process, I saw a chance to clarify some make instructions.

- Took out the unnecessary future tense. (Readers use technical documents to perform tasks or gather information. For readers, these activities take place in the present. Therefore, the present tense is appropriate in most cases. -- Read Me First, A Style Guide for the Computer Industry)
- Put the Mac OS X command line closer to the "Linux standard" example -- was a bit buried in the note
- Clarified the note -- I think this is Docker-specific not make targets in general
- Added the bit about running this in the boot2docker shell just cause I skimmed past the phrase at the top about the containerized make. When I ran the make from a standard terminal the error was not exactly "run this build in a container dummy" so it took me a moment to suss out.