[Documentation] Missing progressive migration advises

[LyzioOh]

Hi, @ankane,

Do you get any advise or stratégie to give us in order to adopt blind index without downtime.

We are looking for a solution to compute an attribute blind indexed value, while on the same time still looking for the cleartext version of that attribute.

For example: I have email_address in clear, may i compute the email_address_bidx, and still looking in email_address column when i do a where query ?

Have a Nice Day, thanks for you're work.
@lyzio

[LyzioOh]

We finally found the option, migrating: true. Are you open to a Pull Request in order to improve the Readme ?

[ankane]

Hey @LyzioOh, great point! I added instructions for how to use the migrating option in the commit above. Happy to review a PR if it can be improved.

I added it to a separate section instead of with the initial instructions since the readme is optimized for the case where you have an encrypted attribute and aren't querying it yet.