Documentation: add purpose of migrations to README

[abcoathup]

Two community members have asked in the forum about the purpose of the migrations folder in ZepKit. https://forum.zeppelin.solutions/t/what-actually-happened-with-the-migrations-folder/765/3

The suggestion was to add the purpose of migrations to the README (potentially FAQ section) https://github.com/zeppelinos/zepkit/blob/master/README.md#faq