Document using multiple codecs in the readme

[tflyons]

Is your feature request related to a problem? Please describe.

I'd like the ability to more easily rotate keys by being able to add a list of valid decryption keys.

Describe the solution you'd like

Change
New(hashKey, blockKey []byte) to
New(hashKey, blockKey []byte, decodeKeys ...[]byte) and add a check that if a cookie cannot be decoded with the hashKey to try with the decode keys until one matches or all fail. This would allow users to have some logic like
New(todaysKey(), nil, tomorrowsKey(), yesterdaysKey()) and rotate keys automatically even in a distributed environment. It would also preserve backwards compatibility with New(hashKey, nil).

Describe alternatives you've considered

There's the obvious, just don't rotate the keys. Or coordinate a failover mechanism by chaining decodes using multiple secure cookies. This seems like a more straightforward approach however.

[tflyons]

Alternative solution: adding methods such as RotateHashIn(hashKey []byte)/RotateHashOut(hashKey []byte)/RotateBlockIn(blockKey []byte)/RotateBlockOut(blockKey []byte)
'In' methods to move a new key into the hashKey/blockKey variable and move the old key into a hash/block slice for decoding. 'Out' methods to remove a key from the slice.