Document way for clients to send non-GET requests

[chrysn]

It appears that the only way to construct a client request without manually splitting the URI (ie. re-implementing parse_coap_url) is using the get method, which precludes the use of different request methods.

Please document how a request POST or PUT message, with possible additional options, is supposed to be created from a URI.

[Covertness]

Thank you for your advice. Now you can find the examples in https://github.com/Covertness/coap-rs/tree/master/examples