Missing mention of newly added `key` command in main help text

[mib-kd743naq]

Version information:

go-ipfs version: 0.4.5-dev-d83a911
Repo version: 4
System version: amd64/linux
Golang version: go1.7.4

Type:

Enhahcement

Priority:

P3

Description:

~$ ipfs help | grep -c key
0

[Kubuxu]

@whyrusleeping making it 0.4.5, it should be quite fast and docs are important.