Upgrade / Migration guide from Pocco's version

[okuuva]

Switching the plugin source from one repo to another isn't as straight forward as it should be. There are subtle gotchas depending on the package manager you use. The most common being the old version of the plugin being cached Somewhere™ and nvim using that instead of the version you'd expect after doing changes to your config.

We should add a short Upgrade / Migration section to readme to ease the transition for people.