Add logging docs

docs/logging.md

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+# Logging
+
+`divnet-rs` logs various things to standard error during a run.
+
+## Log levels
+
+There are multiple log levels.  Here they are from least important to most important.
+
+- `trace`
+- `debug`
+- `info`
+- `warn`
+- `error`
+
+By default `divnet-rs` only prints `info`, `warn`, and `error` messages.
+
+If you want, you can turn on the lower level messages using the `RUST_LOG` [environmental variable](https://www.digitalocean.com/community/tutorials/how-to-read-and-set-environmental-and-shell-variables-on-linux) like this:
+
+```
+RUST_LOG=debug divnet-rs config.toml
+```
+
+That would tell `divnet-rs` to print all `debug` messages plus any messages that are of higher importance (`info`, `warn`, and `error`).
+
+If you want fewer messages, you could use `RUST_LOG=warn` or `RUST_LOG=error`.
+
+## Recommendation
+
+Generally the default log level is fine.  It's also nice since you don't have to set any environmental variables!
+
+If you have very large input files and want to see the most logging, for example to try and estimate the time remaining, you could turn on mode messages with `RUST_LOG=trace` or `RUST_LOG=trace`.  It will put a lot of output though.
+