Adding more emphasis to the need of create a go file before running ensure

[fbidu]

What does this do / why do we need it?

Hello, first-time contributor here!

I was following along dep's tutorial right now and got stuck while trying to add new packages right before running dep init. It turns out I was missing creating a valid .go file beforehand.

After reading through some issues I found some references about the need to create a file before adding the first dependencies. When I went back to the documentation with more attention, I found the part I skipped.

I guess most users pay more attention to the highlighted code parts than to the overall text. I know it was my mistake but one that I think might happen to more users and can easily be prevented by changing the docs a little bit. I considered changing the paragraph structure but I think that a simple emphasis in bold text is enough

What should your reviewer look out for in this PR?

Do you need help or clarification on anything?

Which issue(s) does this PR fix?

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[fbidu]

I signed it!

[googlebot]

CLAs look good, thanks!

[googlebot]

CLAs look good, thanks!

[sdboyer]

Welcome, thank you for the contribution! 🎉 🎉

Yes, this is a rather important point, and it's definitely a gotcha that trips people. i've no qualms about bolding it 😄