Add more usage examples

[bexelbie]

Can you expand on your README a bit and help with use cases for this extension? does it store your SSH keys in the password store and provide them when required to SSH? Is it a way of ensuring your keypasswords are strong by having pass provide them when you unlock your GPG key?

[ibizaman]

An example would be nice indeed. Let's say we want to use an ssh key called test.

pass ssh --fzf

If you don't have any ssh key, it would show an empty fzf screen. Write test and press Enter. This will:

1. Create a passphrase under sshkey-passphrase/test.
2. Create a private/public key pair under ~/.ssh. ~/.ssh/test and ~/.ssh/test.pub.
3. Print to stdout the public key ~/.ssh/test.pub.

The output in my case is:

Creating passphrase in sshkey-passphrase/test
Generating ssh key /Users/me/.ssh/test
Generating public/private rsa key pair.
Your identification has been saved in /Users/me/.ssh/test.
Your public key has been saved in /Users/me/.ssh/test.pub.
The key fingerprint is:
SHA256:YN...80 user@hostname
The key's randomart image is:
+---[RSA 4096]----+
...
+----[SHA256]-----+
ssh-rsa AAAA...pRtw== user@hostname

A subsequent call to pass ssh --fzf and selecting test again would only print the public key to stdout:

ssh-rsa AAAA...pRtw== user@hostname

When you then try to connect to a host using this key, you would call pass --clip sshkey-passphrase/test to access the passphrase.

[ibizaman]

Added a commit with a real example in the README.