Add cli guide for `nebula-cert`

[jasikpark]

❯ nebula-cert -h
Usage of nebula-cert :
Global flags:
-version: Prints the version
-h, -help: Prints this help message

Modes:
ca : create a self signed certificate authority
keygen : create a public/private key pair. the public key can be passed to nebula-cert sign
sign : create and sign a certificate
print : prints details about a certificate
verify : verifies a certificate isn't expired and was signed by a trusted authority.

[jasikpark]

Mentioned in slackhq/nebula#53