readme: Update build instructions

[elcore]

1. What does this change do, exactly?

Update the build instructions to reflect the transition to Go modules. This will not be necessary in Go 1.13, according to https://blog.golang.org/modules2019

2. Please link to the relevant issues.

#2560

3. Which documentation changes (if any) need to be made because of this PR?

n/a

4. Checklist

• I have written tests and verified that they fail without my change
• I have squashed any insignificant commits
• This change has comments explaining package types, values, functions, and non-obvious lines of code
• I am willing to help maintain this change if there are issues with it later

@mholt The PR template does not show up :/

[mholt]

Thanks for thinking of this!