Document migrations

[julianrubisch]

I'm currently writing up a blog post that includes a cursory introduction to Kredis.

In that process I've noticed that the included migration capability is yet undocumented.

As I haven't used it myself yet, I'm not the ideal person to write it up, but I thought I'd make a note of it here for good measure. If anyone could provide me with usage examples, I'd happily set up a doc PR.

[gcimmino]

In my mind migrations should update something like data structure but trying this methods it came out that Kredis::Migration.migrate_all and Kredis::Migration.migrate just copy passed keys. Kredis::Migration.delete_all deletes all passed keys as expected.
You can see some usage example in migration_test.rb but sincerely i don't know if the purpose of migrate* methods was just to copy keys because in migration_test.rb should be tested also the presence of original keys.

[dhh]

If anyone wants to turn this into a PR, please proceed!