Add a configuration section in README.md

PiperOrigin-RevId: 545196016

Author: bjacotg

README.md

@@ -275,6 +275,21 @@ fn always_fails() -> Result<()> {
 }
 ```
 
+## Configuration
+
+This library is configurable through environment variables. Since the
+configuration does not impact whether a test fails or not but how a failure is
+displayed, we recommend setting those variables in the personal
+`~/.cargo/config.toml` instead of in the project-scoped `Cargo.toml`.
+
+### Configuration variable list
+
+| Variable name       | Description                                        |
+| ------------------- | -------------------------------------------------- |
+| GTEST_RUST_NO_COLOR | If set to any value, disables ANSI output from the |
+:                     : failure message. This is useful when the failure   :
+:                     : description is piped to a file or another process. :
+
 ## Contributing Changes
 
 Please read [CONTRIBUTING.md](CONTRIBUTING.md) for details on how to contribute