remove vendored event.schema.json from codebase, document where it can be found in the future

api/README.md

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+## Findings about event.schema.json
+
+There are 2 locations where this file can be sourced (a good and a bad one):
+The 2 locations have diverged (of course!)
+
+### sentry-data-schemas
+
+In the sentry-data-schemas repo:
+
+* https://raw.githubusercontent.com/getsentry/sentry-data-schemas/main/relay/event.schema.json
+
+This is MIT-licenced.
+
+The repo contains a setup.py, but:
+
+* the result of that didn't make it to pypi
+* the result of that is Python files (mypy) and does not contain the json file.
+
+### Sentry (the main repo)
+
+In the sentry repo:
+
+* https://github.com/getsentry/sentry/blob/master/src/sentry/issues/event.schema.json
+* https://github.com/getsentry/sentry/blob/6b96e8f0c484/src/sentry/issues/event.schema.json
+
+This is not 'real' Open Source.
+
+### Notes on divergence:
+
+The main point of divergence (other than just the fact that the laws of nature force code to drift apart) is that
+the sentry's codebase has, as per the commmit that adds it:
+
+> added `"project_id"` field (in the API this would have been added from the URL path)
+
+See also the "caveats" section here:
+
+https://github.com/getsentry/sentry-data-schemas?tab=readme-ov-file#relayeventschemajson
+
+In short, the more reasons to just use the "upstream" API.
+
+Said in another way: we act more as the "relay" than as "getsentry/sentry", because we do ingest straight in the main
+process. So we should adhere to the relay's spec.
+
+### Notes on use:
+
+Bugsink, as it stands, doesn't use event.schema.json much.
+
+* We have `--valid-only` as a param on `send_json`, but I appear to have used that only sporadically (back in nov 2023)
+* We _could_ at some point in the future [offer the option to] throw events through a validator before proceeding with
+  digesting. At that point we'll re-vendor event.schema.json (from the sentry-data-schemas repo)
+* Reading this file is useful, but we can do that straight from the source.