Mention caveat in README about IndexedDB outside browser (#4712)
If using the SDK outside the browser with end-to-end encryption,
IndexedDB usage needs to be disabled otherwise it will cause the
Rust code to panic.
See also:
- https://github.com/matrix-org/matrix-js-sdk/issues/4570
- https://github.com/matrix-org/matrix-rust-sdk-crypto-wasm/issues/168
- https://github.com/matrix-org/matrix-rust-sdk-crypto-wasm/issues/195#issuecomment-2661510343
Signed-off-by: Adam Spiers <github@adamspiers.org>