document credential management

[chadwhitacre]

See gratipay/gratipay.com#3998 (comment) for context.

[chadwhitacre]

2d28ef9:

[chadwhitacre]

Successful key management is critical to the security of a cryptosystem. In practice it is arguably the most difficult aspect of cryptography because it involves system policy, user training, organizational and departmental interactions, and coordination between all of these elements.

https://en.wikipedia.org/wiki/Key_management

[chadwhitacre]

Time to land this now that gratipay/gratipay.com#3998 is in and going out.