Document Migrating from Mcrypt (removed in PHP 7.2)

[jamalo]

Documentation for migrating from mcrypt & Re-encrypt encrypted database data. PR #210 ? @regularjack

[gsdevme]

Worth speaking about the mcrypt polyfill also which could help in the migration perhaps.

https://github.com/phpseclib/mcrypt_compat

[jamalo]

any update? @schmittjoh