Mention changes in V3 regarding "routes" in docs

[mhsattarian]

Consider mentioning the changes regarding routes in the migration guide from v2 to v3.

I assume many snowpack users are developing SPA application requiring client-side routing, and upgrading to version 3 would break the previous behavior of serving project:

previously below configuration would serve up the project as static and client-side routings were fine:

on version 3 the same configuration as above wouldn't work alone, and it requires changes below: