Add comparison to passlib

At a quick glance

Both packages support

• Auto-updating KDFs

Passlib has

• Customizable preference of KDFs (work factors not customizable)
• More stars on github
• A on go report card (https://goreportcard.com/report/github.com/hlandau/passlib)

Passhash has

• 100% code coverage
• Customizable work factors for KDFs (preference of KDFs not customizable - at least not yet...)
• Customizable password policies
• Audit log support
• Customizable storage of credentials (e.g. load credentials mastered written by an existing system)
• A+ on go report card (https://goreportcard.com/report/github.com/dhui/passhash)