Undocumented feature flags

Nobody has claimed this yet.

I just tried to build `matrix-sdk` as a dependency in my program with the following invocation in `cargo.toml`:

```toml
matrix-sdk = { version = "0.7.0", features = ["e2e-encryption"], default-features = false }
```

This led to the following error showing up in my terminal, upon attempted `cargo run`:

```
error: failed to run custom build command for `matrix-sdk v0.7.0`

Caused by:
process didn't exit successfully: `C:\Users\Alyssa\Miscellanea\Programming\Rust\matrix-chat-exporter\target\debug\build\matrix-sdk-48c5aca6c0ec4c49\build-script-build` (exit code: 1)
--- stderr

┏━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━┓
┃ error: one of the features 'native-tls' or 'rustls-tls' must be enabled ┃
┗━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━┛
```

...which is strange, because neither `native-tls` nor `rustls-tls` are listed as settable feature flags on [the `matrix-sdk` crates.io page](https://crates.io/crates/matrix-sdk). So it seems like the documentation there is out-of-date, missing some feature flags which in fact exist (and, moreover, are required for a successful build).

(Adding `rustls-tls` to the features list, after seeing this error message, let me subsequently compile without further issue; my report is purely of a documentation problem, not of anything wrong on a deeper technical level.)

Start by checking the matrix-sdk feature list shown on crates.io against the feature flags exposed by the package configuration. Document native-tls and rustls-tls, including that one is required when default-features is false, and verify that the documented configuration builds successfully with the reported Cargo.toml setup.

Written by the indexing model from the issue text.