fix(grammar): improves readability of first two sections

[lukepeterson]

• This PR makes a number of grammatical improvements to the first two chapters of Learn Go With Tests. There is nothing too crazy in here yet - just adding commas where they are missing and some small rewrites to clarify.
• I'm also updating the .gitignore as I'm on a Mac and have dozens of .DS_Store files in my fork. 😄

[quii]

Thanks, Luke. Just as an FYI, do a Google for "global gitignore"—you can have a gitignore on your system that applies to all git repos. I, too, use a Mac most of the time, and this makes this issue disappear.