docs: added info on updating the schema

packages/tl/UPDATING.md

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+## updating checklist
+
+updating a schema and keeping up to date with telegram is a bit of a headache, 
+so here's a checklist to help me (and maybe you) out:
+
+### common steps
+- run `pnpm run fetch-and-gen`
+- look at the diff in `diff.json` for `long` types, make sure to update `data/int53-overrides.json` if needed
+- run `pnpm run fetch-and-gen` again if there were any changes (note that this will overwrite diff.json file so you may want to have a copy of it)
+- run `pnpm run -w lint:tsc` to make sure everything typechecks, if not – fix it.
+
+### type-specific steps
+
+#### message-related updates
+
+if there were changes to message-related `Update`-s, make sure to update `_fetchMissingPeers` 
+in updates manager to accomodate for any new fields.
+
+note that this also aplies to any types used inside them, including but not limited to:
+- `messageAction*`
+- `messageMedia*`
+- `messageReply*Header`, `messageForwardHeader`, etc.
+- `messageEntity*`
+
+#### peer-related updates
+
+if there were changes to `user` or `channel`, make sure to update `service/peers.ts` min updating
+logic to accomodate for any new fields.