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.
Fred, I actually find the use of "root" ambiguous in the original text.  Can you clarify this for me?  By root was it meant the root directory or the root user?  

In my rewrite, I assumed the meaning was the "root" user because 1) isn't that the purpose of sudo in this case and 2) the make command as it appears in the example could only be run in the root directory where the makefile is located (if I recall correctly; I'm not at my desk to test).

The way the note reads to me now is "do not build in the root of the Docker repo" and "do not use sudo"  -- is that the correct interpretation?  Maybe the preposition "under" is the issue:

On Mac OS X, Docker make targets such as `build`, `binary`, and `test` should not be built as root. Therefore, you shouldn't use sudo when running these commands on OS X.