Update upgrade documentation for SQlite

[fastlorenzo]

@fastlorenzo - it doesn't look like there's any mention of sqlite in the new values.yaml, and no mention in the README or migration guide; does that mean it's not supported?

Originally posted by @UbiquitousBear in #218 (comment)

We should add some information on the README that sqlite is used by default, as well as updating the UPGRADE guide for sqlite.

[dominch]

It would be nice to have something more about pv in readme,
for now it's not that clear what data and where is located, also why they are not several pv

[migs35323]

i was looking into the new releases and from the documentation i was assuming SQLITE was gone
"The embedded MySQL database has been removed from this chart."

i did read it wrong, i tought this was talking about the sqlite DB