[docs] db warning

README.md

···
 
 you can see [random.wisp.place](https://tangled.org/did:plc:dfl62fgb7wtjj3fcbb72naae/random.wisp.place) for an example on how to use hydrant.
 
+**WARNING: *the db format is not stable yet.*** it's in active development so if you are going to rely on the db format being stable, don't (eg. for query features, if you are using ephemeral mode this doesn't matter for example, or you dont mind losing your existing backfilled data in hydrant if you already processed them.).
+
 ## vs `tap`
 
 while [`tap`](https://github.com/bluesky-social/indigo/tree/main/cmd/tap) is designed as a firehose consumer and simply just propagates events while handling sync, `hydrant` is flexible, it allows you to directly query the database for records, and it also provides an ordered view of events, allowing the use of a cursor to fetch events from a specific point in time.

src/types.rs

···
     pub status: RepoStatus,
     pub rev: Option<DbTid>,
     pub data: Option<IpldCid>,
+    /// this is when we *ingested* any last updates
     pub last_updated_at: i64, // unix timestamp
     #[serde(borrow)]
     pub handle: Option<Handle<'i>>,