rust: document that HttpFgbReader won't work with https urls by default

[msalib]

I stumbled on this -- if you make a simple app using flatgeobuf that doesn't depend directly on reqwest@0.11.0, it will fail to handle https urls with this error message:

Error: http error `error sending request for url (https://blah/blah/blah.fgb): error trying to connect: invalid URL, scheme is not http`

That error originates in hyper. I think the issue is that http-range-client has an optional dependency on reqwest and so without that, you'll get a client that has no support for HTTPS. That's...really odd. I'd suggest either:

• document clearly that HttpFgbReader will not read HTTPS urls unless you add an explicit dependency on the same version of request that flatgeobuf's http-range-client depends on, or
• add a tls or http feature to flatgeobuf and document that in the README.

[pka]

That's not a good behaviour. IMO we should add the TLS feature per default when using the http feature.

[michaelkirk]

I agree. Rather than add more documentation, I think it's preferable to just do what people expect to be done anyway. For the same reason reqwest enables https by default, so should http-range-client. People fully expect an http client to be able to speak tls by default without needing to read documentation and pass special options.

I've opened a PR that does this here pka/http-range-client#4