Document environment variables all in one place

docs/src/environment.md

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+# Environment Variables
+
+## Configuration
+
+Set `TASKCHAMPION_CONFIG` to the location of a configuration file in order to override the default location.
+
+## Terminal Output
+
+Taskchampion uses [termcolor](https://github.com/BurntSushi/termcolor) to color its output.
+This library interprets [`TERM` and `NO_COLOR`](https://github.com/BurntSushi/termcolor#automatic-color-selection) to determine how it should behave, when writing to a tty.
+Set `NO_COLOR` to any value to force plain-text output.
+
+## Debugging
+
+Both `ta` and `taskchampion-sync-server` use [env-logger](https://docs.rs/env_logger) and can be configured to log at various levels with the `RUST_LOG` environment variable.
+For example:
+```shell
+$ RUST_LOG=taskchampion=trace ta add foo
+```
+
+The output may provide valuable clues in debugging problems.