to add some docs for git-credential-keepassxc get command for non git use cases

[muravjov]

I've found out about this nice utility while looking for a way to get KeepassXC credentials in scripts without entering master password every time (like ssh agent).

The classical way is to

$ keepassxc-cli show <db> <entry>

but it requests master password every time. More about the issue is at keepassxreboot/keepassxc#1483

Using subj like

printf "url=https://example.com\nusername=foo\n" | git-credential-keepassxc get

or

(cat << 'EOF'
url=https://example.com
username=foo
EOF
) | git-credential-keepassxc get --json | jq .

does what I need but figuring out it from README.md is not easy.

Can you pls add some docs about what stdin should be for git-credential-keepassxc get explicitely.