Mention LSP_TEST_LOG_STDERR in README

Author: lukel97

lsp-test/README.md

@@ -41,7 +41,7 @@ Whilst writing your tests you may want to debug them to see what's going wrong.
 You can set the `logMessages` and `logStdErr` options in `SessionConfig` to see what the server is up to.
 There are also corresponding environment variables so you can turn them on from the command line:
 ```
-LSP_TEST_LOG_MESSAGES=1 cabal test
+LSP_TEST_LOG_MESSAGES=1 LSP_TEST_LOG_STDERR=1 cabal test
 ```
 
 ## Developing