Minor Improvement to Using Cork Section

[Bubblemelon]

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gpg --receive-keys 9CEB8FE6B4F1E9E752F61C82CDDE268EBB729EC7
gpg --verify cork.sig cork

GPG versions earlier than 2.1 will generate the following error:
gpg: Invalid option "--receive-keys"

I would like to add a snippet saying:

Use following as alternative:

gpg2 --receive-keys 9CEB8FE6B4F1E9E752F61C82CDDE268EBB729EC7
gpg2 --verify cork.sig cork

Using gpg2 will generate the same output as indicated by the immediate example as specified in the docs:

gpg: Signature made Thu 19 Apr 2018 03:33:40 PM PDT
gpg:                using RSA key 9CEB8FE6B4F1E9E752F61C82CDDE268EBB729EC7
gpg: Good signature from "CoreOS Application Signing Key <security@coreos.com>" [unknown]
gpg: WARNING: This key is not certified with a trusted signature!
gpg:          There is no indication that the signature belongs to the owner.
Primary key fingerprint: 18AD 5014 C99E F7E3 BA5F  6CE9 50BD D3E0 FC8A 365E
     Subkey fingerprint: 9CEB 8FE6 B4F1 E9E7 52F6  1C82 CDDE 268E BB72 9EC7

[bgilbert]

--keyserver keys.gnupg.net --recv-keys 9CEB8FE6B4F1E9E752F61C82CDDE268EBB729EC7 will work with both gpg and gpg2. (gpg appears not to have a default keyserver, thus the first option.) Want to make a PR?

[Bubblemelon]

@bgilbert you're right! I suggested using gpg2 in case we wanted to keep to the --receive-keys 9CEB8FE6B4F1E9E752F61C82CDDE268EBB729EC7 command format.

I could easily change the current:
gpg --receive-keys 9CEB8FE6B4F1E9E752F61C82CDDE268EBB729EC7
to
gpg --keyserver keys.gnupg.net --recv-keys 9CEB8FE6B4F1E9E752F61C82CDDE268EBB729EC7

Which will work for older gpg versions, and I'll include the alternative using gpg2 in the doc as well.

[bgilbert]

My point was that if we use command-line arguments that work with both versions, we can avoid the gpg2 alternative entirely, simplifying the doc.

[Bubblemelon]

@bgilbert Okay got it!