docs: add relnotes for v2.0.0

relnotes/v2.0.0.md

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+## Major changes in 2.0.0
+
+- Database backend switched from LevelDB to SQLite. There is a migration on
+  first launch which can be lengthy for larger setups. The new database is
+  easier to understand and maintain and, hopefully, less buggy.
+
+- Deleted items are no longer kept forever in the database, instead they are
+  forgotten after six months. If your use case require deletes to take
+  effect after more than a six month delay, set the
+  `--db-delete-retention-interval` command line option or corresponding
+  environment variable to zero, or a longer time interval of your choosing.
+
+- Modernised command line options parsing. Old single-dash long options are
+  no longer supported, e.g. `-home` must be given as `--home`. All serve
+  options are now also accepted as environment variables. See  `syncthing
+  serve --help` for details.
+
+- Rolling hash detection of shifted data is no longer supported as this
+  effectively never helped. Instead, scanning and syncing is faster and more
+  efficient without it.
+
+- A "default folder" is no longer created on first startup.