Added notes to README to clarify version compatibility

[jlandahl]

Motivation

Version compatibility with the OpenTelemetry crates is potentially confusing to new adopters, as the current version of this crate is 0.26, but it is not compatible with 0.26 of the OpenTelemetry crates. A naive developer might simply set the versions for all the dependencies to 0.26 in their own project, which results in some very confusing compiler errors that take some research to figure out (ask me how I know!).

Solution

I added a new section titled "Compatibility with OpenTelemetry crates" just below the "Overview" section. It includes a link to the discussion in #170 as well as a special note about the current 0.26 versus 0.25 situation.

While I was at it, I updated the dependencies in the Cargo.toml example in the "Basic Usage" section and ensured that the example code still works with the updated versions.

I also added to the "Visualization example" section, including a link to the opentelemetry-otlp.rs example and a minimum Cargo.toml example for using that code in a separate project. I simplified the dependencies there to include only the minimum features necessary for the example to work.

[djc]

Wanted something slightly different, proposed in #172, mentioning you as a co-author on the relevant commit. Thanks!