README: simplify installation instructions with go get.

[cpu]

Prior to this commit the installation instructions in the README had you
use git clone and then go build. This can be collapsed into one step
using a more idiomatic go get command. This commit updates the README
accordingly.

Edit: I also snuck in a commit ( 670ed03 ) to document the two locations a
config file can be placed. Happy to split this off into a separate PR if preferred.

[coveralls]

Coverage remained the same at 92.373% when pulling c9f9a9c on cpu:cpu-simplify-readme into 830cceb on joohoi:master.

[MrBMT]

@cpu The command in your initial pull request should be:
go get github.com/joohoi/acme-dns/...
Rather than:
go get https://github.com/joohoi/acme-dns/...

Otherwise you'll be presented with the following error:

package https:/github.com/joohoi/acme-dns/...: "https://" not allowed in import path

Cheers

[cpu]

@cpu The command in your initial pull request should be:
go get github.com/joohoi/acme-dns/...

@MrBMT Oops! You're correct :-) That's the go get in my shell history, I made an editorial mistake preserving the https:// URL when I was putting it in the README. Fixed in c9f9a9c, thank you for catching that!

[joohoi]

Thanks for the PR @cpu and for the intial review @MrBMT ;) It's awesome to have someone to check and make the instructions better!